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Further   Researches   Concerning 
Atomic  Weights 


of 


Potassium,    Silver,    Chlorine,    Bromine, 
Nitrogen,  and  Sulphur 


THEODORE  WILLIAM  RICHARDS 

IN    COLLABORATION   WITH 

ARTHUR    STAEHLER,    GEORGE    SHANNON    FORBES, 
EDWARD   MUELLER,  AND   GRINNELL  JONES 


WASHINGTON,  D.  C: 

Published  by  the  Carnegie  Institution  of  Washington 

April,  1907 


# 

,^  ^   Li    tJ 


CARNEGIE  INSTITUTION  OF  WASHINGTON 
PUBLICATION  No.  69 


CONTRIBUTIONS  FROM  THE  CHEMICAL  LABORATORY 
OF  HARVARD  COLLEGE 


Press  of  Byron  S.  Adams 
washington,  d.  c. 


CONTENTS 


A  Revision  of  the  Atomic  Weight  of  Potassium  ;  the  Analysis  of  Potas- 
sic  Chloride.    By  T.  W.  Richards  and  Arthur  Staehler. 

Page 

Introduction 7 

Preparation    of   the    Material 9-14 

Potassic    Chloride 9 

Argentic  Nitrate 13 

Silver 13 

Nitric  and   Hydrochloric  Acids 14 

Water 14 

The  Drying  and  Weighing  of  the  Potassic  Chloride 14-16 

The  Precipitation  and  Weighing  of  Argentic  Chloride 17-21 

The  Ratio  of  Potassic  to  Argentic  Chloride 21 

The  Ratio  of  Potassic  Chloride  to  Silver 22 

Discussion   of   Final   Results 22-23 

Summary 24 

A  Revision  of  the  Atomic  Weight  of  Potassium  ;  the  Analysis  of  Potas- 
sic Bromide.    By  T.  W.  Richards  and  Edw^ard  Mueller. 

Introduction 27 

The  Preparation  of  Materials     28-36 

The  Source  of  the  Potassium 28-30 

Bromine 30 

Potassic   Bromide 31-34 

Silver 34-35 

Nitric    acid     35 

Water 35 

The  Laboratory 35 

Utensils 35 

The  Drying  and  Weighing  of  Potassic  Bromide 36-38 

The  Precipitation  and  Weighing  of  Argentic  Bromide 38-40 

The  Ratio  of  Argentic  Bromide  to  Potassic  Bromide 40 

The  Determination  of  the  Silver  Needed  for  Precipitation 41-42 

The  Ratio  of  Potassic  Bromide  to  Silver 42 

The  Atomic  Weight  of  Potassium 43 

Summary 44 

The    Quantitative    Synthesis    of    Argentic    Nitrate    and    the    Atomic 
Weights  of  Nitrogen  and  Silver.    By  T.  W.  Richards  and  G.  S.  Forbes. 

Introduction 47 

Preparation   of   Pure   Materials 47-49 

Nitric  Acid 47 

Silver 48 

Water 49 

Air 49 

The  Synthesis  of  Argentic  Nitrate      50-54 

3 


4  CONTENTS. 

Page 

The  Purity  of  the  Fused  Argentic  Nitrate 55-63 

Tests  for  Dissolved  Air 55-57 

Tests  for  Retained  Water 57-60 

Tests  for_  Insoluble  Impurities 60 

Tests  for  Ammonic  Nitrate     61 

Tests  for  Nitrite 62 

Tests  for  Free  Acid 62 

The  Final  Result  and  Its  Relations  to  the  Values  of  Atomic  Weights  of  Nitro- 
gen and  Silver 64-65 

Summary 65 

The  Molecular  Weight  of  Argentic  Sulphate  and  the  Atomic  Weight  of 
Sulphur.    By  T.  W.  Richards  and  Grinnell  Jones. 

Introduction 69 

Preliminary  Experiments 72-74 

The  Preparation  of  Pure  Materials 74-75 

Sulphuric   Acid      74 

Argentic  Sulphate 74 

Hydrochloric  Acid 75 

The  Fusion  of  Argentic  Sulphate 75-80 

The  Conversion  of  Argentic  Sulphate  into  Chloride 80-85 

The  Final  Results 86-87 

The  Atomic  Weight  of  Sulphur 87-88 

Summary 88 


A  Revision  of  the  Atomic  Weight  of  Potassium 


The  Analysis  of  Potassic  Chloride 


By  Theodore  William  Richards  and  Arthur  Staehler 


Contributions  from  the  Chemical  Laboratory  of  Harvard  College 


A  Revision  of  the  Atomic  Weight  of  Potassium. 


THE  ANALYSIS  OF  POTASSIC  CHLORIDE. 


INTRODUCTION. 

The  atomic  weight  of  potassium  is  a  chemical  constant  of  unusual  inter- 
est and  significance.  Standing,  as  it  does,  in  the  middle  of  the  series  of 
five  atomic  weights  of  the  most  electro-positive  metals  —  substances 
which  exhibit  in  a  highly  marked  degree  both  similarity  and  progressive 
change  in  properties  —  this  number  must  be  of  unusual  importance  in  the 
search  for  that  undiscovered  mathematical  relationship  which  undoubtedly 
exists  between  these  constants.  Moreover,  the  atomic  weight  of  potassium 
is  essentially  bound  up  with  the  atomic  weights  of  oxygen,  chlorine,  and 
silver,  this  group  forming  a  fundamental  basis  in  the  calculation  of  nearly 
all  the  other  atomic  Aveights ;  so  that  a  change  in  the  atomic  weight  of 
potassium  has  a  widel}^  ramified  eflfect  on  the  whole  table  of  atomic 
weights.  If  any  further  evidence  of  this  fact  were  needed,  the  recent 
paper  of  R.  W.  Gray^  would  furnish  it. 

The  recent  investigation  upon  the  atomic  weight  of  sodium  by  one  of 
us  in  conjunction  with  Dr.  R.  C.  Wells^  showed  conclusively  that  Stas's 
work,  upon  which  rested  earlier  knowledge,  was  somewhat  at  fault  in 
several  respects.  The  most  serious  of  these  errors  were,  first,  the  existence 
of  impurity  in  Stas's  silver;  secondly,  the  fact  that  in  his  work  solid  salt 
was  dropped  into  the  argentic  solution,  causing  occlusion  of  salt;  and 
thirdly,  inadequate  knowledge  concerning  solutions  of  argentic  chloride. 
•  In  vi»w  of  these  errors  it  seemed  not  impossible  that  a  similar  revision  of 
potassium  miglrt  likewise  yield  slightly  lower  results  for  potassium  than 
had  been  found  by  this  master  of  exact  analysis.  For  this  reason  the 
present  investigation  was  undertaken. 

Before  describing  the  present  work  a  brief  historical  review  of  previous 
investigations  may  not  be  out  of  place.  The  determinations  may  be 
divided  into  two  groups  —  the  first  group  including  all  of  these  deter- 
minations in  which  the  molecular  weight  of  a  potassic  halide  was  found 
by  decomposition  of  a  chlorate,  bromate,  or  iodate,  and  the  second  group, 
those  in  which  data  for  determination  of  the  relation  of  potassium  to 

iR.  W.  Gray,  Journ.  Chem.  Soc.  (Lond.),  Trans.,  89,  1175  (1906). 
^Richards  and  Wells,  Carnegie  Inst,  of  Washington  Publication  28;  Jonrn.  Am. 
Chem.  Soc,  27,  459  (1905)  ;  Zeits.  anorg.  Chem.,  47,  56  (1905). 

7 


8  THE  ANALYSIS   OF   POTASSIC   CHLORIDE. 

chlorine,  bromine,  or  iodine  were  obtained.  The  second  group  alone  is 
concerned  with  the  present  work. 

The  early  work  of  Berzelius,  Pelouze,  and  Marignac  need  scarcely  be 
considered  here.^  It  dealt  for  the  most  part  with  the  precipitation  of 
argentic  chloride  from  a  solution  of  potassic  chloride  and  gave  very 
widely  discrepant  results.  Even  the  later  work  of  Marignac  on  chloride, 
bromide,  and  iodide  of  silver  was  of  doubtful  value,  especially  the  last; 
and  accordingly  until  the  present  research  was  undertaken,  the  atomic 
weight  of  potassium  rested  chiefly  on  the  analyses  of  potassic  chloride 
made  by  Stas,  by  Richards  and  Archibald,  and  by  Archibald  alone,  and 
upon  the  analysis  of  potassic  bromide  made  by  Stas. 

As  in  the  case  of  sodium,  Stas^  made  several  series  of  determinations 
of  the  amount  of  silver  needed  to  precipitate  a  known  amount  of  potassic 
chloride.  One  of  these  series  was  made  in  1865,  a  later  one  in  1881,  and 
yet  another  shortly  before  his  death.  The  earliest  series,  although  in 
some  respects  more  careful  and  thorough  than  the  later  ones,  was  greatly 
at  fault,  because  in  it  he  overlooked  the  solubility  of  argentic  chloride. 
Accordingly  of  this  work  only  the  later  need  be  considered.  The  mean 
of  seventeen  analyses  indicated  that  100  parts  of  silver  were  equivalent 
to  69.132  parts  of  potassic  chloride,  corresponding  to  an  atomic  weight 
for  potassium  of  39.130,  if  silver  is  taken  as  107.93  and  chlorine  as  35.473. 
Later,  Richards  and  Archibald^  found  as  a  side  issue  of  a  research  upon 
the  atomic  weight  of  caesium  that  100  parts  of  silver  correspond  to  69.115 
parts  of  potassic  chloride,  and  also  that  100  parts  of  argentic  chloride 
correspond  to  52.022  parts  of  potassic  chloride;  and  still  more  recently, 
Archibald*  found  results  not  very  different.  With  the  values  given  above 
for  silver  and  chlorine  these  results  indicate  a  value  for  potassium  of 
39.123  and  39.128,  respectively,  in  the  case  of  Richards  and  Archibald, 
and  39.122  and  39.135,  respectively,  in  Archibald's  research. 

In  the  case  of  the  bromide  Stas  found  in  an  unusually  varying  series 
of  experiments  that  100  parts  of  silver  needed  110.346  parts  of  potassic 
bromide  for  complete  precipitation,  a  result  not  very  far  from  that  of  the 
less  precise  early  work  of  Marignac.  If  Baxter's  value  for  bromine  is 
accepted  (79.953),°  this  corresponds  to  an  atomic  weight  for  potassium 
of  39.143,  noticeably  higher  than  the  results  from  the  chloride.  At  that 
time  the  discrepancy  did  not  appear,  because  the  atomic  weight  of  chlorine 

^For  a  brief  discussion  of  this  work  Clark's  Recalculation  of  the  Atomic  Weights 
(1897)  p.  41,  may  be  consulted. 

2Stas,  Mem.  Acad.  Roy.  Belg.,.43  (1880)  ;  also  Oeuvres  Posthumes,  edited  by  W. 
Spring.     See  Clarke's  Recalculation  (1897),  p.  42. 

3Richards  and  Archibald  (1903),  Proc.  Am.  Acad.,  38,  443  (1903).  Zeit.  anorg. 
Chem.,  34,  353,  1903. 

^Archibald,  Trans.  Roy.  Soc.  Canada  [2],  10,  III,  47  (1904). 

^Baxter,  Journ.  Am.  Chem.  Soc,  28,  1322   (1906). 


INTRODUCTION.  l> 

was  not  properly  evaluated,  Stas  believing  that  his  work  on  the  chloride 
also  indicated  an  atomic  weight  above  39.14. 

In  view  of  the  fact  that  these  results  differ  among  one  another  to  an 
extent  beyond  a  reasonable  limit  of  experimental  error,  it  seemed  advis- 
able to  investigate  once  more  the  atomic  weight  of  potassium  in  order  to 
detect  the  cause  of  the  discrepancy.  This  appeared  to  be  especially  desir- 
able on  account  of  the  recent  gain  in  knowledge  concerning  the  peculiari- 
ties of  argentic  chloride  in  solution.  Accordingly,  the  present  research 
was  begun,  and  simultaneously  another  concerning  potassic  bromide.  A 
description  of  the  latter  investigation  follows  immediately  after  this  one. 

An  investigation  upon  atomic  weights  naturally  resolves  itself  into 
several  different  portions  —  first,  the  preparation  of  the  pure  material ; 
second,  the  method  of  drying  and  weighing  this  material ;  third,  the 
details  of  the  analysis  and  the  calculation  of  the  results.  These  will  be 
considered  in  order  in  the  following  pages. ^ 

PREPARATION  OF  THE  MATERIAL. 
POTASSIC  CHLORIDE. 

Several  methods  of  preparing  potassic  chloride  were  tried,  the  most 
efificacious  being  adopted.  The  problem,  of  course,  was  to  effect  the 
elimination  of  other  metals  and  other  acids,  especially  those  of  the  same 
groups.  Both  ends  are  most  quickly  and  completely  attained  by  incor- 
porating into  the  process  of  preparation  successive  crystallization  of 
different  salts,  in  order  to  eliminate  by  the  crystallization  of  one  salt  an 
isomorphous  substance  which  might  have  been  retained  during  the  crys- 
tallization of  another  salt.  The  commonest  salts  of  potassium  were 
therefore  studied  in  relation  to  their  fitness  for  the  elimination  of  impur- 
ities in  this  way. 

In  the  first  place,  on  account  of  the  good  results  which  Richards  and 
Wells  had  attained  in  preparing  pure  sodic  chloride  from  recrystallized 
sodic  sulphate  by  precipitation  with  gaseous  hydrochloric  acid,  sulphate 
of  potassium  also  was  early  considered.  This  method  was,  however,  soon 
abandoned,  because  the  relative  solubilities  of  potassic  sulphate  and 
chloride  are  far  less  favorable  for  the  purpose  than  those  of  the  sodium 
salts.  The  method  of  Archibald,  who  converted  sulphate  into  chloride 
through  precipitation  with  baric  chloride,  is  complicated  and  involves  the 
double  work  of  preparing  pure  baric  chloride  as  well  as  pure  potassic 
sulphate.  Moreover,  it  is  not  easy  to  eliminate  the  last  trace  of  baric 
sulphate  from  the  resulting  salt.     This  also  was  rejected. 

^A  somewhat  less  detailed  statement  of  this  work  has  appeared  in  the  Berichte  d. 
deutsch,  ch.  Gesel.,  39,  3611  (1906). 


10 


THE  ANALYSIS  OF   POTASSIC   CHLORIDE. 


Next  potassic  chlorate  was  studied  —  a  salt  which  had  already  been 
used  by  Stas  for  the  preparation  of  pure  potassium  material.  Both  in 
crystalline  form  and  in  solubility  this  salt  is  very  suitable  for  the  separa- 
tion of  sodium  from  potassium  material.  Sodic  chlorate  crystallizes  in 
regular  crystals  which  are  six  times  as  soluble  in  hot  water  and  sixteen 
times  as  soluble  in  cold  water  as  the  monoclinic  crystals  of  potassic 
chlorate.  On  the  other  hand,  the  data  concerning  the  crystalline  form 
and  solubility  of  the  chlorates  of  rubidium  and  caesium  are  not  suffi- 
cient to  enable  one  to  make  any  certain  prediction  with  regard  to  their 
behavior,  and  therefore  experiments  were  made  in  order  to  discover 
whether  or  not  these  salts  could  easily  be  separated  from  potassic  chlorate 
by  recrystallization.  The  four  salts  of  sodium,  potassium,  rubidium,  and 
caesium  were  mixed  together  and  qualitatively  separated  by  fractional 
crystallization.  The  sodium  was  effectively  separated  from  the  crystals; 
but  after  three  careful  crystallizations  the  caesium  line,  although  stronger 
in  the  mother  liquor  than  at  first,  was  still  not  absent  from  the  crystals. 
Indeed,  neither  rubidium  nor  caesium  lines  had  considerably  diminished 
therein.    Accordingly,  this  method  was  also  abandoned. 

Potassic  nitrate  showed  itself  to  be  much  more  satisfactory  in  its 
behavior.  Its  usefulness  did  not  seem  at  first  so  certain  as  to  be  a  fore- 
gone conclusion,  especially  as  Stas  had  not  succeeded  in  preparing  a 
particularly  pure  material  from  this  salt.  Probably  this  failure  was  due 
to  the  circumstance  that  he  carried  out  the  crystallization  in  glass  vessels, 
\\'*hich  of  course  continually  introduced  sodium  and  silica.  The  rather 
inadequate  data  concerning  the  four  nitrates  may  be  well  put  together 
as  follows : 


Solubility    in    100    parts    of    water. 

Crystalline   form. 

Cold. 

Warmer. 

Temperature. 

Parts. 

Temperature. 

Parts. 

CsNOs 

0 

3 

0 

14 

15 

12 
20 

25 

84 

0 

60 

10 

114 

114 

Very  many 

43 

337 

200 

Tetragonal. 
Needles. 
Rhombohedral 
and  rhombic. 
Rhombohedral. 

RbNOg 

KNO3 

NaNOa 

Because  this  series  of  results  is  not  entirely  conclusive,  although 
promising,  a  series  of  crystallizations  was  carried  out  here  also.  This 
series  showed  that  an  admixture  of  the  three  other  nitrates  with  a 
large  excess  of  potassic  nitrate  was  nearly  all  eliminated  by  as  few  as 
two  crystallizations.    The  separation  of  the  sodium  was  especially  marked 


PREPARATION  OF  THE  MATERIAL.  11 

in  the  spectroscopic  tests.  This  method,  therefore,  appeared  very  well 
suited  for  the  preparation  of  pure  potassium  material,  which  might  easily 
be  converted  into  chloride  after  the  other  metals  had  been  eliminated. 

Pure  potassic  nitrate  of  commerce,  obtained  from  Germany,  was  dis- 
solved in  a  little  water,  freed  from  solid  substances  by  filtration,  and 
crystallized  twice  in  Jena  glass  flasks.  The  salt  was  each  time  freed 
from  the  mother  liquor  in  a  porcelain  centrifuge.^  The  resulting  material 
was  four  times  subsequently  recrystallized  in  platinum  vessels,  and  whirled 
in  a  platinum  centrifuge.  The  pure  crystals  were  now  divided  into  two 
parts;  one  part  was  dried  in  a  vacuum  desiccator,  while  the  other  part 
was  subjected  to  yet  six  more  crystallizations  and  whirlings  in  platinum 
vessels. 

It  now  became  a  question  how  this  nitrate  could  best  be  converted  into 
chloride.  The  method  recommended  by  Stas  of  heating  the  salt  with 
pure  ammonium  chloride  involves  not  only  the  preparation  of  this  latter 
salt  in  a  pure  state,  but  also  great  danger  of  contamination  from  the 
vessel,  whether  of  platinum  or  porcelain.  Platinum  is  especially  attacked 
because  of  the  action  of  the  oxychlorides  of  nitrogen  and  of  chlorine.  The 
more  favorable  method  appeared  to  be  to  convert  the  nitrate  into  chloride 
by  repeated  evaporation  with  hydrochloric  acid  in  quartz  dishes.  This 
method  was  found  to  have  two  objections.  In  the  first  place,  dishes 
large  enough  for  the  raw  material  were  not  to  be  had.  Moreover,  the 
tendency  of  the  branching  crystals  to  grow  over  the  edge  of  the  dish 
caused  serious  loss  of  material  and  danger  of  impurity.  Neither  could 
potassic  nitrate  be  heated  to  a  high  temperature  with  ammonic  chloride, 
or  in  the  steam  of  hydrochloric  acid  in  quartz  dishes;  for  there  is 
always  danger  that  the  reaction  4KNO3  +  2Si02  =  2X38103  +  ^NOj  +  O2 
would  occur.2 

Because  Stas  had  never  been  able  to  obtain  a  salt  free  from  silica  when 
working  in  glass  vessels,  we  desired  to  avoid  completely  the  use  of  glass. 
Therefore,  only  one  way  remained,  namely,  to  convert  the  potassic  nitrate 
into  chloride  by  precipitation  with  hydrochloric  acid  in  a  platinum  dish. 
The  resulting  aqua  regia  must  naturally  attack  the  platinum,  but  we  pre- 
ferred this  impurity  to  that  of  silica,  and  found,  moreover,  that  the  amount 
of  platinum  dissolved  is  very  small.  In  the  presence  of  large  masses 
of  potassic  salts,  aqua  regia,  especially  in  the  cold,  dissolves  platinum 
but  little.  For  example,  in  one  experiment  in  which  about  half  a  kilo- 
gram of  potassic  nitrate  was  treated  with  hydrochloric  acid  gas,  only 
three-tenths  of  a  gram  of  platinum  was  converted  into  the  form  of 
potassic  chlorplatinate.     In  subsequent  crystallizations  the  amount  was 

iRichards,  Journ.  Am.  Chem.  Soc,  27,  105  (1905). 
^Richards  and  Archibald,  loc.  cit. 


12  THE  ANALYSIS  OF   POTASSIC   CHLORIDE. 

very  much  less,  because  the  great  mass  of  the  nitric  acid  was  eliminated 
by  the  first  crystallization.  After  the  third  recrystallization,  the  diphe- 
nylamin  reaction  failed  to  reveal  the  presence  of  any  retained  nitrate. 

The  precipitation  was  carried  out  in  the  following  manner :  The  often 
recrystallized,  very  pure  potassic  nitrate  was  dissolved  in  the  least  possible 
quantity  of  cold  water,  contained  in  a  platinum  dish,  packed  in  ice. 
Slowly,  in  order  to  prevent  warming,  hydrochloric  acid  gas  was  run  into 
this  solution.  The  gas  was  supplied  by  warming  a  pure  concentrated 
solution,  and  was  passed  into  the  nitrate  solution  through  an  inverted 
platinum  funnel.  The  potassic  chloride,  which  appeared  somewhat  yellow 
from  its  slight  platinic  impurity,  was  whirled  in  the  platinum  centrifuge, 
and  dissolved  in  purest  water.  Potassic  chlorplatinate  is  practically  insol- 
uble in  a  concentrated  solution  of  the  chloride,  as  the  law  of  concen- 
tration effect  and  the  theory  of  ionization  predicts ;  hence  the  impurity 
was  easily  separated  upon  a  small,  pure  filter.  The  clear,  colorless  solution 
was  again  saturated  with  hydrochloric  acid,  and  the  same  round  of  oper- 
ations repeated.  Upon  a  third  repetition  of  this  process,  the  nitric  acid 
was  eliminated  and  the  salt,  as  a  rule,  was  wholly  colorless  in  consequence. 
On  no  occasion,  even  with  large  amounts  of  materials,  did  perceptible 
color  remain  after  five  treatments.  No  specimen  was  subjected  to  less 
than  seven  such  recrystallizations,  while  some  was  passed  yet  five  times 
more  through  the  precipitation  and  centrifugal  drying.  Assuming  that 
each  time  about  three-quarters  of  the  ordinarily  adhering  mother  liquor 
was  removed  by  the  centrifugal  separation,  these  crystals  must  have  been 
as  free  from  heteromorphous  substances  as  if  they  had  been  recrystal- 
lized many  thousand  times  in  the  ordinary  fashion.  It  will  be  shown  that 
all  the  samples  gave  the  same  results  upon  analysis,  hence  further  puri- 
fication according  to  this  method  was  needless.  The  salt  was  found  to 
be  wholly  free  from  sodium  by  a  sensitive  spectroscopic  test.  It  was  in 
every  case  freed  from  acid  by  a  single  recrystallization  from  the  purest 
water,  and  by  fusion  in  the  manner  to  be  described  later. 

Seven  different  samples  were  prepared  after  this  fashion  from  varying 
raw  material.     These  are  given  in  the  list  below. 

(a)  The  pure  potassic  material  ("C.  P."  of  German  preparation)  was 
recrystallized  as  nitrate  six  times,  and  precipitated  as  chloride  seven  times. 
This  salt  after  the  final  recrystallization  from  pure  water  was  dried  in  a 
platinum  dish  at  120°  in  an  electric  oven.  The  salt  was  snow-white,  and 
was  as  clear  as  water  when  fused,  as  indeed,  were  all  the  other  samples. 

(b)  This  sample  was  similar  to  a,  but  obtained  by  the  evaporation  of 
the  final  aqueous  mother  liquor  decanted  from  a.  The  mother  liquor  from 
b  was  rejected. 


PREPARATION  OF  THE  MATERIAL.  13 

(c)  Five  extra  precipitations  with  hydrochloric  gas  (or  twelve  in  all) 
yielded  this  salt,  which  was  prepared  for  analysis  in  the  same  way  as  a. 

(d)  This  salt  bore  the  same  relation  to  c  that  b  bore  to  a,  being  essen- 
tially identical  with  c. 

(e)  For  this  sample  and  the  next  one  the  nitrate  was  recrystallized 
twelve  times,  and  the  chloride  precipitated  seven  times. 

(/)  From  the  final  mother  liquor  of  e,  f  was  prepared,  as  b  followed 
after  a. 

(2)  This  sample  was  exactly  like  a,  except  that  the  raw  material  came 
from  an  entirely  different  source. 

ARGENTIC   NITRATE. 

The  nitrate  of  silver  used  for  precipitating  chlorine  in  the  analy^sis  was 
crystallized  three  times  from  pure  dilute  nitric  acid  solution  and  each 
time  centrifugally  freed  from  mother  liquor.  Before  each  analysis  the 
aqueous  solution  of  the  salt  was  tested  in  the  nephelometer^  for  a  possible 
trace  of  argentic  chloride,  and  no  material  was  ever  used  which  showed 
to  this  exceedingly  sensitive  test  the  least  trace  of  impurity.  For  preser- 
vation in  a  pure  state  it  was  kept  in  a  tight  desiccator  over  potash. 

SILVER. 

The  metallic  silver  used  in  the  research  was  made  from  argentic  nitrate 
which  had  been  six  times  recrystallized.  The  metal  was  precipitated  as 
fine  powder  by  ammonic  formate,-  melted  on  pure  lime,  and  further 
purified  by  electrolysis.^  The  beautiful  crystals  thus  prepared  were  fused 
in  a  stream  of  pure  electrolytic  hydrogen  and  finally  in  a  vacuum  of 
0.1  mm.  The  metal  was  supported  on  a  boat  of  the  purest  lime,  prepared 
from  calcic  carbonate  precipitated  from  the  nitrate  for  this  purpose. 
The  boat  was  provided  with  several  compartments,  each  of  which  held 
enough  silver  for  one  analysis,  in  order  to  avoid  the  possible  introduc- 
tion of  impurity  into  the  metal  by  subsequent  cutting.  It  was  inclosed 
in  a  stout  porcelain  tube,  provided  with  Hempel  water-cooled  stoppers, 
and  was  heated  electrically  in  a  Heraeus  tube-furnace.*  The  vacuum 
was  maintained  by  a  motor-driven  Geryk  oil  air-pump.  Except  for  these 
minor  improvements,  the  preparation  was  essentially  similar  to  the  best 
methods  employed  by  Richards  and  Wells  in  their  often-cited  work,  to 
which  the  reader  is  referred  for  further  details. 

^Richards,  Proc.  Am.  Acad.,  30,  385  (1894)  ;  Zeit.  anorg.  Chem.,  8,  269  (1895). 
2Richards  and  Wells,  Joiirn.  Am.  Chem.  Soc,  27,  475  (1905). 
3J.  L.  Hoskyns  Abrahall,  Journ.  Chem.  Soc.  Trans.    (1892),  660;  also  Richards. 
Proc.  Am.  Acad.,  28,  22  (1893). 
<Heraens,  Zeit.  f.  Elektrochemie,  8,  201   (1902). 


14  THE  ANALYSIS  OF  POTASSIC  CHLORIDE. 

NITRIC   AND    HYDROCHLORIC    ACIDS. 

Very  carefully  tested  preparations  of  commerce  were  used  as  the  raw 
materials  in  the  preparation  of  these  acids.  They  were  redistilled  with 
platinum  condensers  until  they  were  fully  pure  enough  for  the  purposes 
for  which  they  were  needed.  The  hydrochloric  acid  had  been  shown 
in  the  research  on  sodium  to  be  free  from  bromine  or  iodine. 

WATER. 

This  substance,  because  it  is  used  in  larger  quantities  than  any  other, 
must  be  especially  pure.  All  the  water  used,  either  for  preparation  or 
analysis,  was  twice  distilled,  once  with  alkaline  permanganate  and  once 
alone.  For  the  best  work  a  platinum  condenser  was  used.  All  that  was 
needed  for  analysis  was  tested  in  the  nephelometer  immediately  before  use. 

THE  DRYING  AND  WEIGHING  OF  THE  POTASSIC  CHLORIDE. 

The  final  preparation  of  the  salt  for  analysis  is  given  a  special  chapter 
to  itself,  because  the  proper  execution  of  this  feature  is  of  very  great 
importance,  not  less  than  the  purification  of  the  material.  It  is  clearly 
useless  to  separate  0.001  per  cent  of  a  metallic  impurity  if  0.01  per  cent 
of  water  is  allowed  to  remain  in  the  salt. 

Obviously,  superficial  drying  can  not  remove  the  imprisoned  moisture 
in  the  crystals,  hence  they  must  be  fused.^  Except  in  a  few  preliminary 
experiments,  where  a  common  platinum  crucible  was  used,  this  fusion 
was  conducted  in  a  platinum  boat  or  open  bottle  contained  in  a  porcelain 
tube  through  which  a  current  of  nitrogen  was  passing.  The  experience 
in  a  number  of  similar  cases,  that  the  simultaneous  presence  of  oxygen 
and  traces  of  hydrochloric  acid  inclosed  in  the  crystals  is  likely  to  cause 
perceptible  corrosion  of  the  platinum  boat,  was  confirmed  in  this  case; 
hence  an  inert  gas  was  needed.  The  nitrogen  was  prepared  by  the  well- 
known  method  of  Wanklyn,  by  passing  air  charged  with  ammonia 
over  red-hot  copper.  The  excess  of  ammonia  was  carefully  eliminated 
by  washing  the  gas  with  much  dilute  acid.  Because  fused  sodic  chloride 
was  found  to  be  essentially  free  from  dissolved  nitrogen,  we  did  not  think 
it  necessary  to  fuse  this  very  similar  potassium  salt  in  a  vacuum. 

Potassic  chloride,  although  not  so  hygroscopic  as  some  other  salts, 
used  in  similar  researches,  is  nevertheless  far  too  hygroscopic  to  weigh 
safely  when  directly  exposed  to  the  air  of  the  balance  room.  Its  ten- 
dency to  attract  water  was  seen  in  perceptible  crackling  when  the  tube 

iRichards,  Zeit.  phys.  Chem.,  46,  189  (1903). 


THE  DRYING  AND  WEIGHING  OF  THE  POTASSIC  CHLORIDE.  15 

containing  fused  salt  was  opened  in  the  moist  atmosphere  of  the  beaker  to 
be  used  for  the  solution  of  the  salt. 

At  first  the  bottling  apparatus  so  often  used  in  the  Chemical  Laboratory 
of  Harvard  College  was  employed  to  protect  the  fused  chloride  from  mois- 
ture during  the  determination  of  its  weight,  the  chloride  being  contained  in 
a  platinum  boat  protected  by  a  glass-stoppered  weighing  bottle.  Because, 
however,  the  weighing  room  was  not  wholly  constant  in  temperature  or 
in  moisture,  time  and  trouble  were  needed  to  obtain  exact  weighings,  even 
although  the  apparatus  was  always  weighed  by  substitution  against  a 
similar  empty  boat  and  bottle.  The  use  of  quartz  weighing  bottles  proved 
to  be  no  advantage,  indeed,  if  anything,  a  disadvantage.  In  either  case, 
if  time  enough  were  taken,  satisfactory  weighings  could  be  made;  but 
as  time  was  especially  precious,  another  device  was  used  which  wholly 
overcame  the  trouble. 

This  effective  and  satisfactory  device  consisted  in  the  use  of  small 
weighing  bottles  of  platinum,  shaped  like  the  long  Lawrence  Smith 
crucible,  which  were  closed  by  small  platinum  capsules,  fitting  into  the 
bottles  like  ground  stoppers.  One  of  these  bottles,  containing  the  potassic 
chloride,  was  supported  by  a  loop  of  platinum  wire  in  an  inclined  porce- 
lain tube,  and  above  it  in  the  tube  was  placed  its  platinum  stopper.  Glass 
stoppers  fitted  into  each  end  of  the  porcelain  tube,  which  was  encircled 
by  a  suitable  furnace.  After  the  potassic  chloride  had  been  dried  for 
a  long  time  at  a  high  temperature,  just  barely  fused  in  nitrogen,  and 
cooled  in  a  current  of  pure  dry  air,  the  stopper  was  shaken  into  place 
and  the  platinum  bottle  quickly  removed  and  placed  in  a  desiccator. 
This  arrangement  is  essentially  similar  to  the  bottling  arrangement  of 
Richards  and  Parker,  except  for  the  additional  advantages  that  glass 
is  wholly  eliminated  and  that  the  inclosed  air-volume  in  the  tube  is  much 
smaller.  Probably  the  burnished  platinum  joint  is  not  as  tight  as  the 
ground-glass  joint;  but  it  is  amply  tight  enough  to  prevent  any  appre- 
ciable diffusion  of  moisture  during  the  very  brief  exposure  of  the  tube. 
This  was  evident  from  the  constancy  of  the  weighings,  which  were  made 
by  substitution  against  a  similar  empty  platinum  bottle  kept  in  a  similar 
desiccator.  The  weighings  were  very  quickly  performed  and  were  trust- 
worthy. Before  being  used,  the  platinum  bottles  were  repeatedly  ignited 
with  pure  sublimed  ammonic  chloride  in  order  to  remove  iron ;  they 
remained  satisfactorily  constant  in  weight  during  subsequent  experiments, 
the  one  used  for  the  fusion  losing  only  0.3  mg.  in  fourteen  experiments, 
or  on  the  average  0.02  mg.  in  a  single  experiment.  In  one  experiment, 
where  oxygen  was  present  during  the  fusion,  as  much  as  0.3  mg.  was 
taken  from  the  boat,  the  platinum  being  plainly  visible  on  solution.  This 
was  of  course  not  used  for  analysis. 


16  THE  ANALYSIS  OF  POTASSIC  CHLORIDE. 

All  weighings,  whether  of  this  salt  or  of  silver  or  argentic  chloride, 
were  made  by  substitution  in  the  manner  already  described  in  other 
papers.^  The  balance  was  a  short-armed  instrument  made  by  Troemner, 
of  the  type  adopted  in  other  similar  investigations;  it  was  used  only  for 
this  work  and  that  to  be  described  afterward.  The  weights  were  of 
platinized  brass,  carefully  standardized  according  to  the  usual  Harvard 
method.^ 

All  weighings  were  reduced  to  the  vacuum  standard.  Thus  from  every 
apparent  gram  of  silver,  0.00003  gram  was  subtracted,  and  to  every 
apparent  gram  of  argentic  chloride,  0.000071  was  added,  if  the  tempera- 
ture was  20°  and  the  pressure  normal.  Assuming  the  density  of  potassic 
chloride  to  be  1.995,  the  similar  correction  for  this  salt  was  +  0.000456. 
Changes  in  temperature  and  pressure  occasionally  caused  slight  but 
usually  inessential  changes  in  these  corrections.^ 

The  wholly  colorless,  transparent,  fused  salt  was  dissolved  in  the  purest 
water  in  a  covered  beaker  of  Jena  glass  under  a  tightly  fitting  bell-jar. 
The  solution  was  always  perfectly  clear,  except  in  the  rejected  case  already 
cited  and  one  other  similar  one.  After  this  solution  had  been  very  thor- 
oughly washed  away  and  used  for  analysis,  the  platinum  weighing  bottle 
was  heated  for  a  short  time  in  another  crucible,  and  weighed  in  prepara- 
tion for  another  analysis.  The  constancy  of  these  weighings  has  already 
been  discussed. 

The  details  of  preparation  having  been  described,  the  analytical  methods 
themselves  must  be  indicated.  The  problem  was,  as  in  the  case  of  sodium, 
to  determine  the  amount  of  chlorine  present,  the  weight  of  potassium 
being  found  by  difference.  Both  of  the  previously  used  methods  for 
finding  the  amount  of  chlorine  were  used,  and  the  details  are  so  much 
like  those  discussed  by  Richards  and  Wells*  that  much  may  be  assumed 
as  understood.  For  a  full  understanding  of  the  present  work,  that  upon 
sodium  should  be  read  in  connection  with  it. 

The  two  methods  for  the  determination  of  chlorine,  namely,  the  weigh- 
ing of  the  precipitated  argentic  chloride  on  the  one  hand,  and  the  dis- 
covery of  the  equivalent  amount  of  silver  on  the  other,  are  discussed 
below  in  order. 

^For  example  see  Richards  and  Rogers,  Zeit.  anorg.  Chem.,  10,  19  (1895)  ;  also 
Richards  and  Wells,  Jotirn.  Am.  Chem.  Soc,  27,  465  (1905). 

^Richards,  Journ.  Am.  Chem.  Soc, 22,  144  (1900).  These  weights  had  an  average 
density  of  8.3. 

3Richards  and  Wells,  Journ.  Am.  Chem.  Soc,  27,  465  (1905). 

^Loc  cit. 


THE  PRECIPITATION  AND  WEIGHING  OF  ARGENTIC  CHLORIDE.  17 

THE  PRECIPITATION  AND  WEIGHING  OF  ARGENTIC  CHLORIDE. 

For  purpose  of  analysis,  the  solution  of  potassic  chloride  was  carefully 
and  completely  transferred  to  a  large  Jena  glass  Erlenmeyer  flask  with  a 
finely  ground  glass  stopper,  and  diluted  to  the  volume  of  a  liter  or  more. 
To  this  in  the  dark-room  under  the  red  light  was  added  exactly  the  calcu- 
lated weight  of  argentic  nitrate,  never  more  concentrated  than  one-fiftieth 
normal ;  and  the  mixture  was  shaken  for  a  short  time  in  order  to  aggre- 
gate the  great  mass  of  the  precipitate.  Because  of  the  absence  of  an 
excess  of  argentic  nitrate,  the  danger  of  the  occlusion  of  this  salt  was 
slight.  On  the  following  day  the  supernatant  liquid  was  usually  quite 
clear,  and  to  it  now  was  added  the  excess  of  perhaps  0.05  gram  of  argentic 
nitrate  needed  for  complete  precipitation.  This  method  of  treatment 
was  found  to  be  more  satisfactory  than  the  immediate  addition  of  all  the 
argentic  nitrate  at  once,  as  in  this  case  long  and  tiresome  shaking  and 
washing  of  the  precipitate  was  needed  to  eliminate  wholly  the  occluded 
argentic  nitrate.  Thus  was  one  of  the  chief  difficulties  met  by  previous 
experimenters  almost  entirely  overcome.  It  was  shown  in  the  paper  on 
sodium  that  the  simplest  test  for  the  presence  of  argentic  nitrate  is  the 
appearance  of  the  argentic  chloride  after  fusion,  even  a  small  trace  of 
nitrate  causing  a  perceptible  gray-violet  cloudiness.  In  all  cases  when  the 
above  precaution  was  used  the  fused  mass  was  perfectly  clear  and  trans- 
parent. This  is,  however,  an  anticipation  of  the  later  part  of  the  process ; 
the  collection,  ablution,  and  desiccation  of  the  precipitate  should  first  be 
described. 

In  preliminary  experiments,  the  Gooch  perforated  platinum  crucible 
with  asbestos  mat  was  used  with  all  the  precautions  previously  adopted 
in  this  laboratory.  In  this  way  it  is  certain  that  good  results  may  be 
obtained ;  but  the  necessary  second  filtration  of  the  filtrate  in  order  to 
collect  the  traces  of  disintegrated  asbestos  is  a  tiresome  process.  On  this 
account  preliminary  experiments  were  made  with  Gooch  crucibles  pro- 
vided with  a  smooth  and  burnished  mat  of  platinum  sponge,  as  prepared 
by  Heraeus.^  Such  crucibles  are  named  by  him  after  Neubauer,  but 
might  more  properly  be  called  Gooch-Munroe  crucibles.  In  these  tests 
it  was  found  that  such  crucibles  as  a  matter  of  fact  answer  very  well 
for  the  purpose  in  hand  if  certain  special  precautions  are  taken.  It 
is  not  permissible  to  ignite  them  at  a  high  temperature,  unless  the  pre- 
cipitate also  is  to  be  ignited  at  a  high  temperature.  A  crucible  brought 
to  constant  weight  at  150°  was  found  to  lose  0.15  mg.  on  ignition,  prob- 

^Zeit.  Angew.  Chem.,  14,  923  (1901).  A  number  of  experimenters  have  recom- 
mended and  used  a  mat  of  platinum  sponge  in  a  Gooch  crucible,  but  Munroe  was 
probably  the  first.  J.  Analvt.  Chem.,  2,  241  (1888).  Also  Chem.  News.,  58,  101. 
1885. 


18  THE   ANALYSIS  OF   POTASSIC   CHLORIDE. 

ably  because  of  loss  of  previously  adsorbed  water.  Moreover,  when  a 
dilute  solution  of  argentic  nitrate,  potassic  nitrate,  and  nitric  acid,  such  as 
remains  after  an  analysis,  was  passed  through  the  filter,  and  this  was  five 
times  washed  with  water,  and  dried  at  150°,  yet  0.15  more  was  gained, 
probably  due  to  adsorbed  salts.  Very  thorough  washing  after  this 
brought  the  crucible  back  to  its  original  weight  after  drying  at  150°. 
•  These  experiments  showed  that  the  washing  of  the  platinum  sponge 
must  be  very  thorough,  and  that  a  definite  temperature  must  always  be 
used  for  drying.  Other  experiments  showed  that  constant  weight  could 
always  be  obtained,  if  these  conditions  were  fulfilled. 

It  is  well  known  that  argentic  chloride  sometimes  clings  to  a  platinum 
surface,  and  always  shrinks  much  on  drying.  On  this  account  the 
somewhat  delicate  upper  surface  of  the  sponge  was  protected  by  a  closely 
fitting  disk  or  diaphragm  of  platinum  punched  with  many  fine  holes. 
This  diaphragm,  merely  laid  upon  the  top  of  the  sponge  at  the  base  of  the 
crucible,  was  easily  loosened  with  the  precipitate,  and  formed  an  effectual 
protection  for  the  sponge.  After  this  was  done,  no  platinum  was  ever 
lost  from  the  upper  surface ;  and  none  was  ever  carried  away  mechanically 
from  below  in  the  wash-water. 

In  spite  of  the  presence  of  the  disk,  small  particles  of  argentic  chloride 
always  clung  here  and  there  to  the  sponge.  As  a  possible  means  of 
removing  these  when  preparing  for  a  new  analysis,  potassic  cyanide  was 
tested.  It  was  found,  however,  that  not  only  was  this  salt  adsorbed  by 
the  sponge,  but  that  in  the  presence  of  air  platinum  was  dissolved  even 
to  the  extent  of  a  milligram  —  a  fact  not  new,  but  none  the  less  pertinent. 
Thiosulphate  was  not  tried,  because  sulphur  was  an  impurity  little  wished. 
Finally,  while  ammonia  for  a  short  time  was  inefficient,  this  liquid,  when 
concentrated  and  applied  for  twelve  hours  or  more,  dissolved  every  trace 
of  the  silver  salt  without  harming  the  crucible. 

The  technique  of  the  Gooch-Munroe  crucible  having  thus  been  mas- 
tered, this  utensil  was  used  with  satisfaction  as  a  means  of  collecting 
and  weighing  the  precipitate  in  hand.  The  latter  was  first  washed  often 
by  decantation,  as  is  recounted  below,  and  finally  on  the  filtering  crucible, 
which  was  dried  to  constant  weight  by  a  temperature  gradually  rising  to 
150°.  After  careful  weighing,  the  main  mass  was  separated  from  the 
clinging  platinum  disk,  and  was  carefully  fused  in  porcelain,  as  has  often 
been  described.  The  accurately  determined  loss  on  fusion,  amounting 
sometimes  to  a  milligram,  was  calculated  from  the  part  to  the  whole, 
and  applied  as  a  correction  to  the  total  weight  of  the  precipitate. 

This  weight,  even  as  thus  corrected,  did  not,  however,  exactly  represent 
the  total  in  weight  of  the  chlorine,  as  some  of  the  argentic  chloride  was 
dissolved  by  the  wash-water.     The  ablution  had  been  conducted  in  three 


THE  PRECIPITATION  AND  WEIGHING  OF  ARGENTIC  CHLORIDE.  19 

Stages,  just  as  in  the  often-cited  investigation  of  sodium.  The  first  stage 
included  the  mother  Hquor  and  first  five  wash-waters,  containing  inten- 
tionally as  much  argentic  nitrate  as  the  mother  liquor,  and  added  to  remove 
the  greater  part  of  the  potassic  nitrate.  This  first  quantity  of  liquid  was 
entirely  free  from  chlorine  in  every  accepted  analysis,  as  was  shown  by 
careful  testing  in  the  nephelometer,  and  so  was  at  once  cast  aside. 

The  second  stage  of  washing  was  conducted  with  very  dilute  nitric 
acid,  and  yielded  0.5  or  0.6  liter  of  a  very  dilute  solution  of  mixed  nitric 
acid  and  argentic  chloride  and  nitrate.  The  traces  of  chloride  which  it 
contained  were  carefully  estimated  in  the  nephelometer  after  addition 
of  excess  of  argentic  nitrate. 

The  third  stage  of  the  washing,  also  conducted  with  pure  water  acidified 
with  nitric  acid  to  prevent  colloidal  irregularities,  yielded  a  liter  or  more 
of  liquid  containing  much  argentic  chloride.  The  analysis  of  this  liquid 
constituted  a  very  important  part  of  the  work,  and  gave  much  trouble. 

In  order  to  be  as  certain  as  possible  of  the  weight  of  chlorine  it  con- 
tained, this  liquid  was  tested  in  five  cases  according  to  two  very  different 
methods.  The  first  of  these  consisted  in  evaporation  of  the  solution 
under  diminished  pressure  and  actual  weighing  of  the  precipitate,  and 
the  second  consisted  in  a  new  modification  of  the  nephelometer  test. 
Although  the  latter  evidently  gave  the  more  satisfactory  results,  the 
former  is  worthy  of  brief  discussion,  because  its  outcome  certainly  repre- 
sents the  maximum  value. 

In  order  to  carry  out  these  two  parallel  quantitative  tests,  the  collected 
wash-water  was  divided  into  two  parts;  and  the  larger  part,  about  90 
per  cent  of  the  whole,  was  evaporated  under  low  pressure  at  50°  in  a 
specially  made  Jena  glass  2-liter  flask  with  a  glass  stopper.  In  the  course 
of  this  evaporation  the  argentic  chloride  began  to  separate  out  when  the 
volume  had  been  reduced  to  about  0.2  liter,  if  2  mg.  of  the  salt  were 
present  —  a  fact  agreeing  well  with  the  recent  observations  concerning  this 
solubility.^  The  dried  residue  was  dissolved  in  a  very  little  ammonia 
freshly  distilled  in  platinum,  and  washed  into  a  small  precipitating  flask, 
where  it  was  reprecipitated  with  excess  of  argentic  nitrate  and  nitric  acid. 
This  trace  of  precipitate  was  collected  and  weighed  on  a  fresh  Gooch- 
Munroe  crucible  in  the  same  way  as  the  larger  mass.  The  results  are 
given  below  in  comparison  with  those  obtained  from  the  same  solutions 
by  the  nephelometer. 

An  important  modification  was  introduced  into  the  nephelometric  deter- 
mination. One  of  us  has  repeatedly  pointed  out  that  in  order  to  obtain 
satisfactory  results  with  this  instrument  the  solution  to  be  tested  and 
that  used  as  the  standard  of  comparison  must  be  treated  in  exactly  the 

^See  for  example  Bottger,  Zeitschr.  phys.  Chem.,  56,  93  (1906). 


20 


THE  ANALYSIS   OF   POTASSIC   CHLORIDE. 


same  way.^  Hence  the  use  of  a  ground-glass  plate^  as  a  standard  of 
comparison  is  a  very  questionable  proceeding.  It  appears  that  the  precipi- 
tation of  argentic  chloride  from  solutions  of  this  salt  differ  perceptibly 
in  mechanism  from  the  precipitation  of  the  same  substance  from  other 
chloride  solutions,  even  when  argentic  nitrate  in  great  excess  is  used 
in  each  case.^  Accordingly,  in  order  to  secure  a  perfectly  satisfactory 
comparison,  both  precipitates  must  be  dissolved  in  ammonia  and  repre- 
cipitated.  The  precipitate  then  appears  in  each  case  in  precisely  the  same 
condition,  and  yields  trustworthy  results. 

The  present  tests  were  carried  out  in  the  following  manner:  Two 
test-tubes  of  precisely  the  same  volume  (0.025  liter)  were  provided.  Into 
one  of  these  was  placed  0.015  liter  of  the  wash-water,  and  into  the  other, 
serving  as  the  standard  of  comparison,  a  like  volume  of  water  containing 
about  as  much  carefully  measured  chlorine  (in  the  form  of  potassic 
chloride)  as  was  present  in  the  first.  Into  each  was  now  run  5  ml.  of  a 
three-hundredth  normal  solution  of  argentic  nitrate.  The  solutions  were 
stirred  with  a  platinum  stirrer,  previously  cleansed  with  ammonia  and  the 
purest  water.  After  five  minutes  the  precipitates  were  well  formed ;  they 
were  then  both  dissolved  with  the  help  of  a  milliliter  of  freshly  distilled 
ammonia,  and  reprecipitated  with  a  slight  excess  of  nitric  acid,  being  filled 
to  similar  marks  near  the  top  of  the  tubes.  The  two  cloudy  solutions, 
thoroughly  stirred,  were  allowed  to  stand  and  compared  optically  in  the 
nephelometer  in  the  usual  way. 

The  following  table  compares  the  results  obtained  by  the  two  methods. 
The  varying  solubility  of  the  halide  in  the  wash-water  was  mainly  due  to 
difference  in  temperature  at  the  time  of  the  ablution. 

The   Comparison  of  Weights   of  Argentic   Chloride   found  N ephelo metrically  and 

by  Evaporation. 


Experiment  No. 

Volume. 

Weights  of  argentic   chloride. 

By  evaporation. 

By  nephelometer. 

Difference. 

10 

11 

12 
13 
14 

Liters. 
1.45 

1.88 
1.18 
0.90 
1.00 

Mg. 
3.96 

3.82 
2.59 
1.70 
1.74 

Mg. 
3.12 

3.22 
1.97 
1.35 
1.45 

Mg. 

—  0.84 

—  0.60 

-0.62 
-0.35 

—  0.29 

^Am.  Chem.  Journ.,  31,  242   (1904);  35,  509   (1906). 
"Wells,  Am.  Chem.  Journ.,  35,  99   (1906). 

^Richards  and  Wells,  Carnegie  Inst.   Pub.  28    (1905)  ;   Journ.  Am.   Chem.   Soc, 
27,  485  (1905). 


THE  PRECIPITATION  AND  WEIGHING  OF  ARGENTIC  CHLORIDE. 


21 


Thus  the  nephelometer  always  indicated  less  argentic  chloride  than 
the  gravimetric  process.  At  first  the  nephelometer  was  suspected,  but 
the  steadily  diminishing  difference  between  the  two  series  of  results  indi- 
cated that  they  were  approaching  the  point  where  they  would  indicate 
the  same  values.  As  the  nephelometric  treatment  was  invariable,  but 
on  the  other  hand  the  evaporating-flask  might  easily  be  attacked,  it 
seemed  probable  that  the  decreasing  difference  exhibited  by  the  figures 
was  due  to  something  added  to  the  weight  of  the  precipitate  from  the 
latter  source.  Apparently,  as  is  reasonable,  the  flask  was  less  and  less 
attacked  as  it  continued  in  use. 

For  this  reason  the  gravimetric  results  were  wholly  rejected,  and  the 
nephelometric  ones  alone  used  in  the  calculation  of  the  final  results. 


THE  RATIO  OF  POTASSIC  TO  ARGENTIC  CHLORIDE. 

The  final  results  of  the  series  of  analyses  discussed  above  are  recorded 
in  the  following  table.  A  number  of  preliminary  experiments  are 
omitted  from  this  table,  since  it  is  clear  that  no  doubtful  or  imperfectly 
executed  experiment  should  find  a  place  in  such  a  table  of  final  data. 
The  list  is  nearly  consecutive,  however,  as  but  few  experiments  met  with 
misfortune  after  the  processes  had  been  mastered.  The  original  number- 
ing of  the  experiments  is  retained ;  they  were  recorded  in  the  notebook  in 
chronological  order.  All  the  argentic  chloride  referred  to  in  this  table 
was  clear  and  colorless;  a  fact  which  is  one  of  the  best  proofs  that  it 
was  free  from  argentic  nitrate  or  from  organic  dust. 

Final  Series  of  Determinations  of  the  Ratio  KCl:  AgCl. 


No.  of 
Analysis. 

Preparation 
of  KCI. 

Weight   of 
fused  KCl 
in  vacuum. 

Weight  of 

fused  AgCI 
in  vacuum. 

AgCI  :  KCl 
=  100.000  :  X. 

Atomic 
Weight  K   if 
CI  =  35.473. 

13 
14 
15 
20 
31 

Average 

a 
a 
a 
b 
b 

Grams. 

4. 36825 
5. 56737 
6. 41424 
3.27215 
4. 83028 

Grams. 

8. 3986 

10. 7038 

12. 3323 

6.  2913 

9.  2870 

52.012 
52. 013 
52.  012 
52.011 
52.011 

39. 114 

39. 115 
39. 114 
39. 112 
39.112 

52.0118 

39. 1134 

This  result  will  be  discussed  after  the  next  series  has  been  presented. 
It  has  a  probable  error,  calculated  by  the  method  of  least  squares,  of  only 
about  0.0004 ;  accordingly  further  repetition  was  deemed  unnecessary. 


22 


THE   ANALYSIS  OF   POTASSIC  CHLORIDE. 


THE  RATIO  OF  POTASSIC  CHLORIDE  TO  SILVER. 


In  this  series  of  experiments,  weights  of  the  purest  silver  equivalent  to 
entirely  new  portions  of  fused  potassic  chloride,  as  calculated  from  the  pre- 
ceding table,  were  dissolved  in  nitric  acid,  and  the  two  equivalent  solutions 
were  mixed.  Great  care  was  used.  The  presence  of  an  excess  of  silver 
or  of  chlorine  was  then  determined  with  the  nephelometer,  exactly  accord- 
ing to  the  details  of  manipulation  adopted  by  Richards  and  Wells,  whose 
accounts  should  be  consulted  for  particulars.  The  table  below  contains 
the  final  results ;  the  preliminary  practice  experiments  are  omitted  for  the 
same  reason  as  before. 

Final  Series  of  Determinations  of  the  Ratio  Ag:  KCl. 


No.  of 

Preparation 

Weight  KCl 

Weight  Ag 

Ratio  Ag  :  KCl 

Atomic 

Weight  of  K  if 

CI  =  35.473. 

Analysis. 

of  KCl. 

in  vacuum. 

in  vacuum. 

=  100.000  :  X. 

Grams. 

Grams. 

19 

a 

3. 88074 

5. 61536 

69. 109 

39.117 

21 

a 

7. 44388 

10.77156 

69. 107 

39.114 

24 

C 

5. 00681 

7.24514 

69. 106 

39. 113 

27 

e 

5. 04833 

7. 30515 

69.107 

39.114 

30 

e 

8. 19225 

11.85412 

69. 109 

39.117 

32 

d 

4. 99795 

7. 23230 

69. 106 

39.113 

33 
Average 

f 

5. 16262 

7.47042 

69.107 

39. 114 

69. 1073 

39. 1145 

In  this  case  the  "probable"  error  is  even  somewhat  less  than  before, 
being  under  0.0004. 


DISCUSSION  OF  FINAL  RESULTS. 

Thus  two  results  have  been  obtained,  giving  for  the  atomic  weight  of 
potassium  the  value  39.113  by  reference  to  argentic  chloride,  and  the 
value  39.114  by  reference  to  pure  metallic  silver.  The  close  agreement 
of  these  results  is  an  important  evidence  of  their  verity,  and  a  striking 
confirmation  of  the  new  atomic  weight  of  chlorine  found  by  Richards  and 
Wells.  The  atomic  weight  of  chlorine  is  very  simply  calculated  from  the 
results  of  the  two  series  above  as  follows:  CI  =:  (69.1073/52.0118  — 
1.00000)  107.93  =  35.475  — very  near  the  most  likely  value,  35,473. 

Such  difference  as  exists  is  probably  due  to  the  slight  remaining  trace 
of  occlusion  of  foreign  salts  by  the  argentic  chloride.  In  the  present  case 
this  source  of  error  was  eliminated  more  successfully  than  ever  before, 
hence  the  agreement  between  the  two  values  of  the  atomic  weight  of 
potassium  was  closer  than  usual. 


DISCUSSION   OF   FINAL   RESULTS.  23 

It  may  be  noted  that  the  difference  between  the  new  value  and  the 
old  value  of  Stas  is  somewhat  less  than  in  the  case  of  sodium.  Stas's 
results  with  potassic  chloride  led  him  to  the  value  39.146,  about  0.032 
higher  than  the  new  result,  while  with  sodic  chloride  the  difference  was 
0.042.  A  large  part  of  this  difference  in  each  case  is  due  to  Stas's  error 
in  the  atomic  weight  of  chlorine ;  corrected  for  this  error,  the  differences 
become  0.016  and  0.026,  respectively.  The  differences  are  to  be  referred 
to  the  same  causes  in  this  case  as  in  the  others  — ■  namely,  to  Stas's  incom- 
plete knowledge  concerning  solutions  of  argentic  chloride,  to  his  practice 
of  dropping  solid  salt  into  the  precipitating  solution,  and  to  the  presence 
in  his  preparations  of  traces  of  impurity  taken  from  containing  vessels. 
In  the  case  of  potassium  one  or  more  of  these  errors  must  have  been 
less  than  in  the  case  of  sodium. 

The  results  of  Richards  and  Archibald,  although  very  few  in  number 
and  not  intended  to  figure  in  a  discussion  of  this  kind,  were  somewhat 
better  —  probably  because  solid  salt  was  never  used  directly  in  precipita- 
tion. Assuming  the  present  research  to  yield  the  true  value,  their  errors 
in  the  two  series  were,  respectively,  0.009  and  0.014.  The  result  of 
Archibald  alone  for  the  ratio  of  silver  to  potassium  chloride  was  about 
the  same  amount  (0.008)  different  from  the  present  value;  but  his  result 
for  the  other  ratio  was  less  satisfactory,  having  an  error  of  0.021,  even 
greater  than  Stas's.  These  differences  are  not  surprising,  because  less 
was  known  at  that  time  than  at  present  concerning  the  behavior  of 
argentic  chloride  in  solution. 

Although  the  present  paper  presents  strong  evidence  that  the  atomic 
weight  of  potassium  is  really  as  low  as  39.114,  more  remains  to  be  done. 
It  would  have  been  desirable  to  have  used  also  some  other  wholly  different 
method  of  preparing  potassic  chloride,  and,  moreover,  to  have  evaporated 
large  samples  of  the  salt  in  nitrogen  in  order  to  discover  a  possible  non- 
volatile residue.  It  is  very  doubtful  if  these  additional  experiments  would 
have  altered  the  present  result,  especially  considering  the  precautions 
taken  in  the  work  and  the  result  of  the  following  research;  but  never- 
theless it  is  planned  to  pursue  these  matters  further. 

Even  when  every  conceivable  precaution  is  taken,  a  single  salt  is  not 
an  adequate  basis  for  the  certain  decision  of  an  atomic  weight.  For 
this  reason  a  parallel  investigation  on  potassic  bromide  was  simultaneously 
in  progress  at  the  Chemical  Laboratory  of  Harvard  College.  The  next 
communication,  describing  this  other  research,  must  be  considered  in 
connection  with  the  work  which  has  just  been  described.  As  will  be  seen, 
excellent  confirmation  of  the  present  work  is  afforded  by  the  work  with 
the  bromide. 


24  THE  ANALYSIS  OF   POTASSIC  CHLORIDE. 

SUMMARY. 

This  investigation  concerning  the  quantitative  composition  of  potassic 
chloride  resembled  in  many  respects  the  recent  investigation  of  Richards 
and  Wells  on  sodium. 

In  several  details,  however,  improvements  were  introduced  which 
effected  a  considerable  saving  of  time  and  a  perceptible  gain  in  accuracy. 

The  precautions  necessary  for  the  accurate  use  of  the  Gooch-Munroe 
perforated  crucible  were  ascertained ;  its  employment  was  found  to  be 
advantageous. 

Platinum  weighing  bottles  with  conical  ground-platinum  stoppers  were 
used  instead  of  boats  and  glass  tubes  for  weighing  the  potassium  salt. 

Occlusion  of  argentic  nitrate  by  the  precipitated  chloride  was  dimin- 
ished by  allowing  the  latter  to  stand  for  a  long  time  in  a  solution  contain- 
ing neither  excess  of  silver  nor  excess  of  soluble  chloride,  and  by  adding 
more  argentic  nitrate  only  after  the  precipitate  had  assumed  a  fairly  per- 
manent condition  of  aggregation. 

The  nephelometric  estimation  of  small  amounts  of  suspended  argentic 
chloride  was  increased  in  accuracy  by  redissolving  in  ammonia  both  of  the 
opalescent  precipitates  to  be  compared  and  reprecipitating,  in  order  to 
equalize  the  conditions. 

As  final  results,  the  outcome  of  twelve  experiments,  100.000  parts  of 
silver  were  found  to  correspond  to  52.0118  parts  of  potassic  chloride,  and 
100.000  parts  of  argentic  chloride  were  found  to  correspond  to  69.1073 
of  this  salt. 

The  corresponding  values  for  the  atomic  weight  of  potassium  (if  silver 
is  assumed  to  be  107.930  and  chlorine  35.473)  are  39.1134  and  39.1145,  in 
unusually  close  agreement. 


II 

A  Revision  of  the  Atomic  Weight  of  Potassium 


The  Analysis  of  Potassic  Bromide 


By  Theodore  William  Richards  and  Edward  Mueller 


Contributions  from  the  Chemical  Laboratory  of  Harvard  College 


A  Revision  of  the  Atomic  Weight  of  Potassium. 


THE  ANALYSIS  OF  POTASSIC  BROMIDE. 


INTRODUCTION. 

The  foregoing  quantitative  study  of  potassic  chloride  by  Dr.  Arthur 
Staehler  and  one  of  the  present  authors  affords  strong  evidence  that 
the  atomic  weight  of  potassium  is  about  39.114,  shghtly  lower  than  the 
value  based  upon  the  work  of  Stas.  The  authors  fully  appreciated,  how- 
ever, that  the  investigation  of  a  single  compound  is  not  enough  to 
establish  a  chemical  constant  so  important  as  this,  and,  accordingly,  the 
present  investigation  was  prosecuted  simultaneously.  It  was  expected 
that  the  two  investigations  might  either  support  one  another  or  else,  by 
affording  incompatible  results,  lead  to  the  discovery  of  a  constant  error  in 
one  or  the  other,  and  thus  pave  the  way  for  further  advance  in  knowledge. 
As  will  be  seen,  the  work  on  the  chloride  was  satisfactorily  confirmed  by 
the  work  on  the  bromide. 

In  the  present  case  the  careful  study  of  potassic  bromide  was  particu- 
larly necessary,  because  there  already  exist  two  concordant  series  of 
experiments  upon  this  substance,  performed  by  the  old  masters  Marignac 
and  Stas,  pointing  to  a  value  in  the  neighborhood  of  39.14,  instead  of  the 
before-mentioned  new  value  39.11.  In  this  case  the  higher  value  for 
potassium  is  not  diminished,  as  in  the  case  of  the  chloride,  by  an  additive 
correction  in  the  atomic  weight  of  the  halogen,  because  Baxter  has 
•shown  Stas's  estimate  for  bromine  to  have  been  nearly  correct.^  Hence 
the  discrepancy  remains  one  too  serious  to  be  tolerated. 

The  careful  study  of  this  work  of  Stas  and  Marignac  affords  convincing 
evidence  that  the  potassic  bromide  used  by  them  for  analysis  was  not 
sufficiently  pure  for  the  purpose.  Stas  admitted  that  some  of  his  prepara- 
tions were  not  even  wholly  soluble  in  water,  and  his  method  of  procedure 
was  such  that  some  of  them  probably  contained  platinum  and  hydroxide. 
Because  speculations  of  this  kind  concerning  work  so  long  past  are  of 
but  little  value,  it  was  clearly  necessary  to  repeat  this  work  with  modern 
care ;  and  the  following  pages  recount  the  details  of  the  repetition. 

^A  brief  review  of  Stas's  and  Marignac's  work  may  be  found  in  Clarke's  "Recal- 
culations" (1897),  p.  47.  Baxter's  work  is  to  be  found  in  Proc.  Am.  Acad.,  42,  201, 
1906;  Journ.  Am.  Chem.  Soc,  28,  1322  (1906).  See  also  Richards,  Trans.  Am. 
Phil.  Soc,  43,  116  (1904). 

27 


28  THE   ANALYSIS   OF   POTASSIC   BROMIDE. 

In  common  with  others  of  the  same  type,  the  investigation  easily  resolves 
itself  into  several  sections  and  will  be  discussed  under  the  following 
heads :  The  preparation  of  materials ;  the  drying  and  weighing  of  potas- 
sic  bromide;  the  determination  of  the  ratio  of  argentic  to  potassic  bro- 
mide ;  the  determination  of  the  ratio  of  silver  to  potassic  bromide ;  and 
the  discussion  of  the  results.  Of  the  laboratory  tasks  the  preparation 
of  the  materials  was  by  far  the  most  laborious  and  puzzling,  while  the 
analytical  work  was  comparatively  simple. 

THE  PREPARATION  OF  MATERIALS. 
THE  SOURCE  OF  THE  POTASSIUM. 

The  task  of  preparing  pure  potassic  bromide,  simple  as  it  may  appear, 
is  by  no  means  an  easy  one.  In  view  of  our  experience,  it  seems  probable 
that  neither  Marignac  nor  Stas  ever  prepared  samples  pure  enough  to 
correspond  with  the  other  precautions  which  they  took.  As  in  the  case 
of  sodic  bromide,^  the  salt  itself  when  once  made  can  not  be  effectively 
purified.  In  this  respect  it  differs  widely  from  the  chloride.  We  found 
after  due  trial  that  the  potassium  and  bromine  must  be  purified  sepa- 
rately in  such  forms  as  to  introduce  no  foreign  matter.  Accordingly, 
the  two  substances,  already  adequately  purified,  were  caused  afterwards 
to  yield  potassic  bromide ;  and  this  salt,  when  recrystallized,  gave  the  final 
substance  for  analysis.  The  purification  of  the  potassium  materials  will 
be  discussed  first. 

It  was  necessary  to  provide  potassium  material  which  should  be  free 
from  the  suspicion  of  introducing  any  impurity  with  the  metal  desired, 
either  basic  or  acid.  Potassic  nitrate,  which  had  served  so  well  in  the  case 
of  the  chloride,  was  not  suitable  for  the  present  purpose,  because  the 
destruction  of  the  nitric  acid  involves  too  great  a  sacrifice  of  laboriously 
prepared  hydrobromic  acid.  After  due  consideration,  potassic  oxalate 
was  tentatively  chosen.  It  was  necessary  to  prove  that  this  salt  can  be 
easily  and  certainly  purified  by  recrystallization,  and  especially  to  show 
that  the  other  alkali  metals  can  thus  be  separated.  The  following  tests 
accomplished  this  proof. 

A  saturated  solution  of  potassic  oxalate  containing  a  purposely  added 
admixture  of  10  per  cent  of  sodic  oxalate  was  crystallized  and  the  crystals 
drained  centrifugally.  By  the  fllame  test,  a  pronounced  difference  was 
noticeable  between  the  amount  of  sodium  present  in  the  mother  liquor 
and  that  in  the  crystals.  A  second  recrystallization  and  whirling  was 
still  more  satisfactory;  the  crystals  gave  no  sodium  test  to  the  eye,  while 

iRichards  and  Wells,  Proc.  Am.  Acad.,  41,  435  (1906)  ;  Zeit.  phys.  Chem.,  56, 
348  (1906). 


THE   PREPARATION   OF   MATERIALS.  29 

the  mother  Hquors  gave  a  distinct  yellow.  This  indicates  that  sodium 
has  a  decided  tendency  into  the  mother  liquors,  and  hence  can  be  sepa- 
rated by  fractional  crystallization. 

Again,  1  per  cent  of  lithium  salt  was  added  to  potassic  oxalate  with 
similar  outcome.  Rubidium  and  csesium  were  tested  likewise.  A  sat- 
urated solution  of  potassic  oxalate  and  a  salt  of  each  of  these  metals  was 
made  of  such  strength  that  the  rubidium  or  csesium  could  easily  be  found 
by  means  of  the  spectroscope.  Each  solution  was  evaporated  until  satu- 
rated, and  cooled ;  and  the  crystals  were  whirled.  The  mother  liquor  gave 
a  test  for  the  impurity,  the  crystals  gave  none. 

No  other  oxalates  were  seriously  to  be  feared,  because  the  insoluble  ones 
would  appear  at  once  and  demand  filtration,  and  soluble  double  oxalates 
could  hardly  be  isomorphous,  and  would  therefore  tend  into  the  mother 
liquors.  Traces  of  calcic  oxalate  in  fact  appeared  in  the  former  fashion, 
and  traces  of  iron  in  the  latter. 

In  view  of  these  satisfactory  results,  it  was  evidently  only  necessary 
to  crystallize  the  potassic  oxalate  often  enough  to  make  the  separation 
complete;  and  since  potassic  oxalate  has  a  fairly  convenient  change  in 
solubility  with  the  temperature,  the  yield  after  a  considerable  number  of 
crystallizations  is  good,  especially  if  pains  are  taken  to  carry  out  the  pro- 
cess systematically. 

Two  very  pure  samples  of  oxalate  were  made,  from  two  different  sources. 
As  one  source  of  material,  a  commercial  sample  prepared  by  Merck  was 
used.  It  had  been  labeled  "Potassium  oxalate,  neutral,  highest  purity," 
and  contained  as  a  matter  of  fact  only  the  traces  of  calcium  and  iron 
already  referred  to,  but  no  lead  or  other  discoverable  impurity.  A  large 
quantity  of  a  solution  of  this  salt  was  filtered  hot  into  a  platinum  dish, 
where  it  was  five  times  systematically  recrystallized,  the  crystals  being 
centrifugally  whirled  each  time. 

Careful  scrutiny  in  the  spectroscope  revealed  in  the  product  no  other 
spectroscopic  lines  than  those  of  potassium,  and  careful  qualitative  tests 
proved  the  absence  of  lead  and  iron.  The  salt  dissolved  without  a  trace 
of  residue  in  water,  and  was  quite  pure  enough  to  serve  as  a  starting 
point  for  further  operations,  to  be  described  under  the  heading  "Potassic 
bromide." 

A  second  sample  of  potassic  material  was  obtained  from  Merck  in  the 
form  of  the  hydroxide,  being  marked  "Potassium  Hydrate,  Chemically 
Pure  Reagent;  Conforms  to  the  standards  of  Dr.  Krauch."  It  was  in 
fact  very  pure,  giving  a  perfectly  clear  solution  both  before  and  after 
neutralization.  It  was  neutralized  with  a  specimen  of  oxalic  acid  which 
had  been  carefully  purified  by  Mr.  G.  E.  Behr,  jr.,  for  other  work.  This 
acid  contained  no  halogens,  and  left  no  residue  on  volatilization  in  plati- 


30  THE  ANALYSIS  OF  POTASSIC  BROMIDE. 

num.  Slightly  more  than  the  equivalent  quantity  of  acid  was  added,  and 
the  oxalate  obtained  was  therefore  slightly  acid.  As  will  be  shown,  the 
acidity  was  later  the  cause  of  much  inconvenience,  and  had  better  have 
been  avoided.  The  oxalate  was  four  times  systematically  recrystallized  in 
platinum  with  centrifugal  draining  each  time.  The  final  product,  like  the 
previous  one,  gave  no  tests  for  impurities  except  a  slight  excess  of  acid ; 
it  was  used  as  the  source  of  potassium  for  one  preparation  of  potassium 
bromide. 

BROMINE. 

Although  the  methods  previously  used  at  Harvard  had  afforded  a 
satisfactory  yield  of  very  pure  bromine,^  it  was  desired  to  add  to  the 
knowledge  on  the  subject  by  testing  another  method.  Through  the  gener- 
osity of  the  Mallinckrodt  Chemical  Works  of  St.  Louis,  Missouri,  and 
also  of  the  Dow  Chemical  Works  of  Midland,  Michigan,  a  large  quantity 
of  potassic  bromate  was  placed  at  our  disposal.  Repeated  crystallization, 
very  kindly  carried  out  for  us  on  a  large  scale  by  these  firms,  had  been 
the  method  used  in  its  purification. 

This  material,  which  served  as  our  starting-point,  was  already  very 
pure,  a  fact  which  was  indicated  by  three  syntheses  of  argentic  bromide 
from  purest  silver  and  potassic  bromide  obtained  by  decomposing  the 
Mallinckrodt  bromate.  These  syntheses,  carried  out  with  all  necessary 
precautions,  gave,  respectively,  57.441,  57.440,  57.441  as  the  percentage  of 
silver  in  argentic  bromide.  According  to  Baxter  the  true  amount  is 
57.445.^  The  slight  difference  indicates  a  trace  of  iodine  rather  than 
chlorine;  but  this  impurity  can  be  easily  expelled,  while  chlorine  is  far 
more  troublesome. 

It  having  been  demonstrated  that  recrystallization  is  an  effective  means 
of  purifying  the  salt  from  chlorate  and  chloride,  a  large  quantity  of 
potassic  bromate  was  three  times  recrystallized  in  porcelain,  the  crystals 
being  centrifugally  whirled  each  time  and  all  mother  liquors  discarded. 
This  gave  an  exceedingly  pure  bromate,  much  purer  than  Stas  could  have 
obtained,^  because  the  centrifugal  draining  makes  so  great  a  difference 
in  the  rate  of  purification.* 

A  large  quantity  of  the  salt  was  decomposed  into  the  bromide  in  a 
platinum  dish  by  the  heat  of  an  alcohol  lamp.  The  potassic  bromate  in 
decomposing  attacked  the  platinum,  and  the  mass  of  bromide  was  slightly 

iSee  Proc.  Am.  Phil.  Soc,  43,  119  (1904),  for  references  and  Stas,  Mem.  Acad. 
Belgique,  43,  II,  38  (1882). 

2Baxter,  Journ.  Am.  Chem.  Soc,  28,  1332  (1906).  See  also  Richards,  Trans. 
Am.  Phil.  Soc,  43,  119   (1904). 

^Untersuchungen,  160. 

^Richards,  Journ.  Am.  Chem.  Soc,  27,  104  (1905). 


THE  PREPARATION  OF  MATERIALS.  31 

brownish  in  color,  but  this  caused  no  inconvenience  in  the  present  case, 
as  the  bromine  was  to  be  subsequently  distilled.  In  order  to  liberate  the 
halogen,  the  bromide  was  treated  in  strong  solution  with  less  than  the 
calculated  quantity  of  bromate  and  an  excess  of  pure  sulphuric  acid.  The 
acid  was  added  drop  by  drop  to  the  mixed  solution  of  salts  in  an  apparatus 
made  entirely  of  Jena  glass,  which  was  kept  surrounded  by  ice-water. 
The  bromine  which  volatilized  on  account  of  the  heat  of  the  reaction  was 
caught  in  a  condenser  under  water;  to  this  was  added  the  remaining 
bromine,  which  had  been  separated  from  the  supernatant  solution  by 
means  of  a  separating  funnel. 

The  bromine  thus  obtained  was  twice  distilled  by  steam  heat  into  water 
in  a  Jena  glass  condenser,  packed  in  ice.  It  was  kept  in  a  well-seasoned 
bottle  with  double  ground-glass  stoppers. 

The  product  was  certainly  free  from  non-volatile  materials,  and  could 
have  contained  only  iodine  and  perhaps  a  trace  of  sulphuric  acid  as 
impurity.  It  will  be  seen  that  these  were  eliminated  in  the  subsequent 
work.  This  was  the  only  sample  of  bromine  prepared ;  it  was  used  in  all 
the  preparations  of  bromide,  as  the  quantitative  results  indicated  that 
it  was  very  pure. 

POTASSIC  BROMIDE. 

The  problem  of  obtaining  potassic  bromide  from  the  oxalate  and  bro- 
mine had  been  carefully  considered  before  these  substances  had  been 
prepared.  Only  two  methods  recommended  themselves  for  the  present 
purpose,  although  the  bromide  can  be  obtained  from  these  two  materials 
by  a  variety  of  reactions,  which  would  ordinarily  be  acceptable.  Here, 
however,  simplicity,  completeness,  and  theoretical  correctness  must  all  be 
fulfilled;  that  the  methods  adopted  really  possessed  these  advantages  in 
their  favor  must  be  admitted  after  careful  perusal  of  the  following  descrip- 
tion of  the  details. 

In  some  preliminary  experiments  it  was  shown  that  by  adding  an  excess 
of  bromine  to  the  oxalate  and  evaporating  off  the  excess,  a  very  pure 
bromide  quite  free  from  oxalate  is  obtained.  Accordingly,  a  quantity  of 
the  first  sample  of  five  times  crystallized  potassic  oxalate  was  treated  with 
an  excess  of  bromine,  the  bromine  being  added  in  small  quantities  from  a 
dropping-funnel.  The  solution  was  held  in  a  quartz  dish,  which  was 
carefully  hooded  to  prevent  entrance  of  dust.  When  the  action  had 
ceased,  the  solution  was  heated  on  a  steam-bath  until  any  traces  of  iodine 
and  all  excess  of  bromine  had  been  expelled.  The  solution  was  found 
to  be  entirely  free  from  oxalic  acid  upon  testing  with  calcic  nitrate  in  a 
solution  faintly  acid  with  nitric  acid.  Although  it  was  wholly  colorless 
while  in  the  quartz  dish,  after  being  concentrated  in  platinum  it  developed 
a  very  faint  yellow  tinge.    Upon  recrystallizing  three  times  this  color  was 


32  THE   ANALYSIS   OF    POTASSIC   BROMIDE, 

finally  concentrated  in  the  extreme  mother  liquor,  and  was  found  to  be 
due  to  a  trace  of  platinum ;  no  iron  was  present.  Even  the  first  crystals 
were  perfectly  white,  the  second  and  third  mother  liquors  also  showed 
no  trace  of  color,  and  only  the  first  of  the  mother  liquors  gave  a  test 
for  platinum. 

The  material  thus  prepared,  after  being  three  times  recrystallized  in 
platinum  with  centrifugal  draining,  was  called  sample  I,  and  served  for 
analyses  1  to  11. 

The  second  sample  of  potassic  bromide  was  prepared  from  the  second 
sample  of  four  times  recrystallized  potassic  oxalate.  The  method  was  the 
same  as  with  the  previous  sample.  An  excess  of  bromine  was  added  to 
the  oxalate  in  quartz ;  when  action  ceased,  the  bromine  was  driven  off  by 
heating  on  a  steam-bath.  However,  on  testing  for  oxalate,  a  perceptible 
quantity  was  found  still  undecomposed.  This  was  unexpected,  in  view  of 
the  fact  that  the  first  preparation  of  bromide  had  given  no  trouble  in  this 
respect.  The  difficulty  was  traced  to  the  presence  of  a  small  percentage 
of  acid  salt  in  the  oxalate ;  the  presence  of  acid  has  an  inhibitory  effect  on 
the  reaction  of  bromine  on  the  oxalate  ion.  Obviously,  the  simplest 
remedy  would  have  been  to  add  pure  potassic  hydroxide ;  but  this  was  not 
added  because  at  that  moment  there  was  none  at  hand.  Moreover, 
because  the  reaction  was  being  performed  in  quartz,  it  was  thought  best 
not  to  use  a  caustic  alkali  for  fear  that  the  dish  might  be  slightly  attacked. 
The  addition  of  a  small  amount  of  very  pure  recrystallized  bromate  to 
this  slightly  acid  solution  served  better,  because  its  use  was  unattended 
with  danger.  Its  employment  was  entirely  effective ;  the  ionized  hydrogen 
was  removed,  and  after  adding  more  bromine  and  heating,  a  solution  of 
bromide  was  obtained  which  contained  no  trace  of  oxalate. 

This  specimen  was  recrystallized  entirely  in  quartz,  in  order  to  prevent 
a  recurrence  of  the  trouble  experienced  with  sample  I,  as  well  as  to  see 
if  the  variation  of  conditions  affected  the  atomic  weight.  Two  crystalli- 
zations were  made  with  centrifugal  draining,  and  every  precaution;  and 
there  was  obtained  finally  a  very  pure,  colorless  product  with  which  were 
made  two  analyses,  Nos.  12  and  13.  That  a  uniformly  pure  material  had 
been  made  is  shown  by  the  close  agreement  of  these  two  analyses  with 
the  mean  of  all. 

As  a  further  precaution  against  accidental  error,  it  was  thought  best 
to  develop  a  method  essentially  different  from  that  just  discussed,  and 
to  prepare  therewith  a  third  specimen  of  bromide.  In  brief,  this  method 
consisted  in  the  separate  preparation  of  pure  ammonic  bromide  and 
potassic  hydroxide,  and  the  production  of  potassic  bromide  by  the  evapor- 
ation of  the  mixed  solutions  of  these  substances.  Thus  can  be  obtained 
a  salt  contaminated  only  with  excess  of  volatile  ammonic  bromide. 


THE  PREPARATION  OF  MATERIALS.  33 

The  preparation  of  pure  amnionic  bromide  was  very  easy.  Ammonia 
essentially  free  from  carbon  compounds  was  redistilled  into  water  in  a 
platinum  dish.  Into  this  was  dropped  the  purest  bromine,  which  fell 
through  the  liquid  into  a  small  porcelain  crucible  resting  on  the  bottom 
of  the  dish.  In  this  way  the  bromine  itself  could  not  come  into  contact 
with  platinum  and  attack  it.  The  reaction  took  place  very  rapidly,  yield- 
ing ammonic  bromide  which  could  have  contained  no  non-volatile  ingre- 
dients, because  all  its  constituents  had  just  been  distilled. 

The  preparation  of  pure  potassic  hydroxide  was  less  easy  to  devise, 
because  more  original;  but  the  execution  was  almost  as  easy.  The 
problem  was  solved  by  the  use  of  an  electrolytic  process  which  will  be 
described  in  greater  detail  elsewhere.  In  brief,  a  saturated  solution  of 
potassic  oxalate  (sample  I)  was  electrolyzed  between  a  pure  mercury 
cathode  and  a  platinum  anode  in  a  porcelain  dish  cooled  with  ice,  the 
current  from  four  storage  cells  being  used.  When  the  amalgam  became 
solid,  the  current  was  stopped,  the  solution  decanted,  and  the  amalgam 
washed,  being  triturated  with  an  agate  pestle  under  water,  till  no  test 
for  oxalate  could  be  obtained.  The  pure  amalgam  was  transferred  to  a 
platinum  dish,  covered  with  water,  and  made  the  anode  of  a  dense  current 
until  only  a  little  of  the  potassium  amalgam  remained  undecomposed. 
Thus  pure  potassic  hydroxide  was  formed.  This  solution  was  poured  into 
the  ammonic  bromide  solution,  the  mixture  was  evaporated  and  crystal- 
lized, and  the  salt  thus  obtained  used  for  analyses  14,  15,  and  16. 

The  bromide  thus  obtained  must  have  been  very  pure.  During  the 
electrolysis,  the  high  concentration  of  the  potassic  oxalate^  would  tend 
to  allow  the  deposition  of  potassium  only.^  At  any  rate,  metals  less  easy 
to  deionize  could  hardly  have  been  set  free.  During  the  later  decomposi- 
tion of  water  by  the  amalgam,  all  metals  with  less  tendency  to  ionize 
would  have  remained  in  the  mercury,  because  some  of  the  amalgam  was 
left  undecomposed.  Any  trace  of  iodine  which  the  bromine  may  have 
contained  must  have  been  driven  off  by  the  fusion  just  before  analysis, 
this  fusion  being  prolonged  in  order  to  expel  ammonic  bromide.  The 
results  of  the  analyses  of  this  sample  indicate  its  essential  identity  with 
the  two  preceding  preparations.  Since  there  was  no  known  source  of 
impurity  in  any  one  of  the  preparations,  this  was  not  surprising. 

Although  our  consistent  employment  of  insoluble  vessels  rendered  the 
presence  of  colloidal  silica  unlikely,  an  effort  was  made  to  test  for  this 
impurity  of  sample  II  by  volatilizing  3  grams  of  it  in  a  stream  of 
nitrogen.  Unfortunately,  the  high  temperature  of  the  electric  furnace 
caused  the  platinum  boat  containing  the  salt  to  weld  to  the  foil  which 

iSee  Bunge,  Berichte,  9,  78  (1876). 

2Berthelot,  Ann.  Chim.  Phys.  [5],  18,  433  (1879). 


34  THE  ANALYSIS  OF   POTASSIC  BROMIDE. 

surrounded  it  in  the  tube,  so  that  no  certain  quantitative  evidence  could 
be  obtained.  Two  minute  black  spots  were  visible  in  the  boat;  nothing 
else  could  be  seen  upon  very  careful  scrutiny.  If  there  were  any  silica 
present,  the  amount  must  have  been  too  small  to  have  had  any  appreciable 
influence  on  the  final  atomic  weights.  Time  and  material  were  lacking, 
hence  the  experiment  was  not  repeated. 

SILVER. 

The  essential  details  of  the  preparation  of  pure  silver,  as  developed  by 
Richards  and  Wells,  were  followed  in  detail,  and  the  reader  is  referred 
to  their  paper,  as  well  as  to  the  preceding  paper  of  Richards  and  Staehler, 
for  the  particulars.  Two  samples  of  silver  were  made;  they  were  found 
to  be  identical  in  quantitative  behavior. 

The  source  of  the  material  used  for  the  first  preparation  was  the  pure 
precipitated  argentic  chloride  remaining  from  the  work  of  Richards  and 
Wells  on  sodium.  It  was  reduced  to  metallic  silver  with  invert  sugar  in 
a  strong  solution  of  sodic  hydroxide.  The  silver  thus  obtained  was 
washed  free  from  soluble  matter  and  dissolved  in  nitric  acid ;  the  argentic 
nitrate  after  crystallization  and  whirling  was  reduced  with  ammonic 
formate,  yielding  a  beautiful  crystalline  mass  of  metal. 

This  pure  silver,  after  very  thorough  washing,  was  fused  on  lime  by 
means  of  a  blast  lamp  whose  tip  had  been  carefully  cleaned.  The  buttons 
which  were  thus  obtained  were  cooled  in  the  reducing-flame,  washed  free 
from  lime,  scrubbed  with  cleaned  sea-sand,  etched  with  strong  nitric  acid, 
and  washed  with  the  purest  water,  to  serve  as  anodes  in  the  electrolytic 
purification  which  was  the  next  step.  The  electrolysis  was  carried  on 
as  already  described  in  previous  similar  Harvard  researches,  and  every 
step  of  the  subsequent  work  may  be  found  in  detail  in  the  preceding  papers 
on  sodic  and  potassic  chloride.^  The  metal  was  finally  fused  as  usual  on 
a  boat  of  the  purest  lime  in  an  atmosphere  of  hydrogen  under  a  tension  of 
50  mm.  The  larger  pieces,  after  etching  with  nitric  acid,  were  cut  into 
smaller  ones  of  convenient  size  with  a  clean  cold-chisel.  They  were  again 
etched  in  order  to  dissolve  all  traces  of  iron,^  scrubbed  with  clean  sand, 
washed  with  a  jet  of  water,  and  etched  yet  again  with  acid,  paying  par- 
ticular attention  to  the  cut  edges.  Finally  the  pieces  were  dried  over  an 
alcohol  lamp  and  kept  in  a  desiccator  over  potassic  hydroxide  for  use. 

This  silver  was  used  both  in  most  of  the  titrations  against  potassic 
bromide  and  as  the  source  of  argentic  nitrate  used  in  the  precipitation  of 
argentic  bromide  to  be  weighed. 

^See  Proc.  Am.  Acad.,  38,  450  (1903)  ;  and  Richards  and  Wells,  Journ,  Am. 
Chem.  Soc,  27,  473  (1905)  ;  Richards  and  Staehler,  preceding  paper. 

^Richards  and  Archibald,  Proc.  Amer.  Acad.,  38,  450  (1903)  ;  Baxter,  Joum.  Am. 
Chem.  Soc,  28,  1329  (1906). 


THE  PREPARATION  OF  MATERIALS.  35 

A  second  sample  of  silver,  prepared  from  somewhat  less  pure  initial 
material,  was  put  through  a  more  elaborate  and  exhaustive  round  of 
purification  which  need  not  be  detailed.  The  preparation  included  five 
crystallizations  as  nitrate  and  two  successive  precipitations  as  metal  by 
formate.  This  silver  was  fused  into  buttons  of  a  size  suitable  for  imme- 
diate use  without  cutting.  They  were  cleaned  by  etching  and  washing, 
dried  and  preserved  free  from  impurity  as  usual,  and  were  used  in  analyses 

"^  ^"^  ^'  NITRIC  ACID. 

For  preliminary  work,  ordinary  "chemically  pure"  nitric  acid  was 
distilled  once,  using  a  platinum  condenser  and  Jena  glass  receiver,  the 
first  third  of  the  acid  being  discarded.  For  the  final  analyses  this  redis- 
tilled product,  already  very  pure,  was  once  again  distilled,  as  before,  the 
first  third  being  rejected.  This  distillate  gave  no  test  with  the  nephel- 
ometer,  after  dilution.  water. 

As  usual  in  such  work,  all  the  water  used  had  been  carefully  purified  by 
double  distillation,  once  from  a  fairly  strong  solution  of  alkaline  potassic 
permanganate,  and  then  alone.  The  distillate  was  caught  in  Jena  glass 
receiving  flasks  which  were  provided  with  special  adapters  to  prevent 
the  access  of  dust.  The  first  and  last  fractions  were  always  rejected,  only 
the  middle  portion  being  used.  The  connection  of  the  Jena  glass  boiling- 
flask  with  the  pure  tin  condenser  was  made  without  rubber  or  cork.^ 
Such  water  gave  every  evidence  of  sufficient  purity,  as  has  been  frequently 

pointed  out.  ^jjg  LABORATORY. 

A  very  essential  precaution  in  every  stage  of  this  work  was  to  effectually 
exclude  hydrochloric  acid.  Accordingly,  all  the  preparations  of  the  bro- 
mide were  carried  out  in  a  private  laboratory,  used  especially  and  only 
for  this  work,  and  wholly  free  from  the  interference  caused  by  large 
classes  or  other  researches.  Great  care  was  taken  not  to  allow  any 
hydrochloric  acid  in  the  room;  indeed,  not  a  drop  was  used  there  during 
the  entire  research.  A  separate  ventilating  fan  prevented  fumes  from 
other  laboratories  from  interfering  with  the  work. 

UTENSILS. 

As  usual,  great  care  was  taken  to  avoid  the  use  of  any  vessels  which 
under  the  given  conditions  might  be  attacked  and  thus  pollute  the  sub- 
stance in  hand.  When  glass  and  porcelain  were  not  harmful,  as  in  the 
early  stages  of  the  preparation  of  silver,  such  receptacles  were  employed ; 
but  in  the  purification  of  the  substance  for  analysis,  platinum  and  quartz 
were  always  used. 

iRichards,  Proc.  Am.  Acad.,  30,  380  (1894). 


36  THE   ANALYSIS  OF  POTASSIC   BROMIDE. 


THE  DRYING  AND  WEIGHING  OF  POTASSIC   BROMIDE. 

For  reasons  already  mentioned  in  the  paper  on  potassic  chloride,  the 
salt  under  investigation  must  be  fused  before  weighing.  Unfortunately, 
potassic  bromide  when  fused  in  air  in  a  platinum  vessel  attacks  the  plati- 
num to  a  considerable  extent.  Although  it  is  shown  later  that  this  action 
does  not  seriously  affect  the  weight  of  silver  needed  to  precipitate  the 
bromine  in  the  salt,  it  was  nevertheless  desirable  to  avoid  all  such  irregu- 
larities. No  such  action  is  observed  when  the  fusion  takes  place  in  an 
atmosphere  of  pure  dry  nitrogen.  Accordingly,  in  all  work  to  be  re- 
counted, the  potassic  bromide  was  protected  in  this  way  during  its  fusion 
in  a  well-seasoned  platinum  boat,  and  was  finally,  by  means  of  the  Harvard 
"bottling  apparatus,"  shut  up  in  a  tight  weighing  bottle  in  an  atmosphere 
of  pure  dry  air.  The  details  have  already  been  adequately  given  in  several 
places,  especially  in  the  account  of  the  recent  work  on  the  atomic  weight 
of  caesium.^  The  only  difference  in  the  present  procedure  was  demanded 
by  the  high  fusing-point  of  potassic  bromide  (750°) ;  on  account  of  this, 
a  porcelain  ignition-tube  was  substituted  for  one  of  glass,  as  in  the  case 
of  calcic  chloride.' 

Even  with  all  possible  care,  however,  the  platinum  boat  was  distinctly 
attacked,  almost  always  losing  in  weight  during  the  fusion.  In  two  of  the 
cases  —  analyses  1  and  14  —  where  the  potassic  bromide  was  fused  for 
some  time  in  the  boat,  the  corrosive  action  was  very  considerable,  causing 
losses  of  weight  of  0,0007  and  0.0014  gram,  respectively.  One  would  be  dis- 
posed to  reject  these  determinations  entirely,  except  for  the  fact  that  their 

average  result,  — -r —  =  1.10317,  is  almost  exactly  identical  with  that  of 

the  average  of  results  5  and  6,  in  which  the  boat  was  scarcely  attacked  at 

KRr 
all,  namely,  — r =  1.10318.    Instead  of  being  rejected,  these  results 

are  therefore  retained  and  used  as  evidence  that  even  a  considerable 
amount  of  dissolved  platinum  has  no  perceptible  effect  on  the  weight  of 
silver  precipitated  by  the  salt.  In  a  majority  of  the  other  cases,  the  change 
in  weight  of  the  boat  during  the  fusion  was  less  than  0.0001  gram.  The 
average  loss  in  analyses  10,  11,  12,  and  16,  in  which  the  argentic  bromide 
was  weighed,  was  less  than  0.00009  gram;  hence  even  if  platinum  had 
been  present  in  the  potassic  bromide  in  a  state  of  fine  division  it  could  not 
have  exerted  any  essential  effect  upon  the  result  of  this  series  of  analyses. 

iRichards  and  Archibald,  Proc  Amer.  Acad.,  38,  451  (1903)  ;  Zeit.  anorg.  Chem., 
34,  362  (1903). 
sRichards,  Joum.  Amer.  Chem.  Soc,  24,  374  (1902). 


THE  DRYING  AND  WEIGHING  OF  POTASSIC  BROMIDE.  37 

As  a  matter  of  fact,  the  potassic  bromide  always  gave  a  perfectly  clear 
aqueous  solution  except  once,  namely,  in  analysis  14.  In  this  case  several 
small  flakes  of  platinum  were  noticed  in  the  salt  both  before  and  after  the 
salt  was  dissolved  in  water. 

The  solutions  were  always  perfectly  neutral  to  phenolphthalein,  a  fact 
which  is  adequate  proof  of  the  retention  of  all  the  bromine  during  fusion, 
and  also  of  the  absence  of  oxalate  from  the  potassic  bromide  crystals. 

The  third  sample  of  potassic  bromide  had  contained  ammonic  bromide, 
which  was  expelled  during  the  fusion.  During  the  expulsion  of  this  salt, 
the  boat  was  placed  near  the  exit  end  of  the  porcelain  tube,  and  there 
gradually  heated  to  redness  in  a  current  of  nitrogen.  During  the  entire 
operation  the  middle  portion  of  the  porcelain  tube  was  kept  red-hot  in 
order  to  prevent  backward  diffusion  of  the  ammonic  bromide,  which  by 
condensing  might  later  cause  a  contamination  of  the  potassic  bromide. 
All  the  ammonic  bromide  having  been  sublimed,  the  boat  was  pushed  into 
the  middle  of  the  tube,  and  the  potassic  bromide  fused  as  described  above. 
There  being  a  possibility  that  even  at  the  fusing-point  of  potassic  bromide 
some  ammonic  bromide  might  still  be  retained,  another  portion  of  the  same 
third  sample  of  bromide  was  fused  under  approximately  the  same  condi- 
tions as  in  the  analyses.  The  solution  of  the  bromide  thus  obtained  was 
submitted  to  the  Nessler  test,  being  compared  with  a  tube  of  pure  water 
containing  the  same  amount  of  reagent.  No  color  was  visible  in  either 
tube,  hence  no  appreciable  amount  of  ammonic  bromide  remained  to  con- 
taminate the  potassium  salt. 

The  salt  thus  prepared  for  analysis  was  weighed  in  its  glass-stoppered 
weighing-bottle  by  substitution,  using  as  the  substituting  tare  a  precisely 
similar  weighing-bottle.  Successive  weighings  of  the  same  specimen  were 
always  practically  identical.  The  balance  and  weights  were  similar  to 
those  used  with  sodic  and  potassic  chlorides,  and  all  the  precautions  were 
the  same.  At  each  weighing  the  heights  of  the  barometer,  the  temperature 
of  the  balance-room,  and  the  humidity  as  indicated  by  a  hygrometer  were 
recorded.  The  density  of  the  displaced  air  could  thus  be  determined  for 
each  case.  The  variations  from  the  average  were,  however,  not  enough  to 
affect  essentially  any  of  the  results,  as  weighings  were  not  made  under 
any  abnormal  atmospheric  conditions.  The  weights  were  of  course  stand- 
ardized by  the  usual  Harvard  method. 

The  specific  gravity  of  potassic  bromide  must  be  known  to  within  five 
one-hundredths  of  a  unit  in  order  to  correct  the  weighings  to  the  vacuum 
standard  with  sufficient  accuracy.  Its  determination^  has  been  undertaken 
with  a  variety  of  methods,  with  a  variety  of  results,  ranging  from  2.20  to 

ipor  the  literature  see  Landolt  u.  Bornstein  (Meyerhoffer).  Physikalisch-Chem- 
ische  Tabellen  (1905),  and  Clarke,  Constants  of  Nature  (part  1),  Macmillan  (1888). 


38  THE    PREPARATION    OF    MATERIALS. 

2.756.  The  more  recent  results  all  tend  towards  the  highest  value,  found 
by  Krichmeyer^  (2.756).  This  appeared  to  be  the  most  probable,  because 
the  usual  errors  cause  low  results;  but  as  this  determination  was  made 
with  only  a  single  crystal,  and  as  the  salt  weighed  in  the  present  research 
had  been  fused,  it  was  thought  best  to  investigate  further.  Accord- 
ingly, a  pure  sample  of  potassic  bromide  was  further  purified  by  being 
three  times  recrystallized  with  centrifugal  draining,  when  it  gave  no  flame 
test  for  sodium.^  This  product  was  fused  in  platinum  and  coarsely 
pulverized ;  its  specific  gravity  was  determined  in  an  Ostwald  pycnometer, 
modified  for  use  with  solids,  using  toluol  as  the  fluid  to  be  displaced.  The 
toluol  had  been  redistilled  and  possessed  a  density  0.8608  at  25°,  referred 
to  water  at  4° ;  8.2568  grams  of  potassic  bromide  displaced  2.6009  grams 
of  the  toluol  at  25°,  a  result  which  indicates  a  density  of  2.73.  Time  was 
lacking  for  a  repetition  of  the  experiment,  but  this  preliminary  result  was 
enough  to  show  that  the  fused  salt  is  essentially  in  the  same  state  as  the 
crystallized  salt.  The  diflPerence  between  the  two  values  is  too  small  to 
affect  the  vacuum  correction ;  the  average  value  2.74  was  used. 

THE  PRECIPITATION  AND  WEIGHING  OF  ARGENTIC  BROMIDE. 

In  order  to  determine  the  weight  of  argentic  bromide  obtainable  from 
the  bromine  in  potassic  bromide,  the  latter  salt  was  precipitated  by  a  very 
slight  excess  of  argentic  nitrate,  both  substances  being  dissolved  in  large 
quantities  of  the  purest  water.  The  method  differed  but  slightly  from 
that  which  had  been  used  recently  in  so  many  other  cases  of  the  same 
kind.  The  argentic  nitrate  was  prepared  from  a  weighed  amount  of 
pure  silver,  but  no  attempt  was  made  in  this  series  to  determine  the  exact 
amount  of  silver  required,  as  it  was  desired  not  to  complicate  the  process 
before  weighing  the  argentic  bromide.  Care  was  taken  to  have  an  excess, 
but  only  a  slight  excess.  The  silver  was  dissolved  in  nitric  acid  with  all 
the  usual  precautions,  and  the  precipitation  was  carried  out  in  orange-red 
light  in  the  dark-room  devoted  to  accurate  work  of  this  kind.  The  potas- 
sic bromide  had  been  dissolved  out  of  the  boat  by  digestion  in  a  large 
platinum  dish,  and  been  transferred  to  a  2-liter  glass-stoppered  Jena 
Erlenmeyer  flask  with  careful  rinsing,  and  every  precaution  to  prevent 
any  gain  or  loss.  After  being  well  shaken,  the  flask  containing  the  silver 
bromide  was  allowed  to  stand  until  the  mother  liquor  had  become  clear. 
The  solution  was  filtered  through  a  platinum  Gooch-Munroe  crucible, 
and  the  precipitate  was  washed  by  decantation,  first  with  an  extremely 
dilute  acid  solution  of  silver  nitrate,  and  finally  with  exceedingly  dilute 

iKrichmeyer,  Z.  physikal.  Chem.,  21,  81  (1896). 
2See  Krichmeyer's  experience,  loc.  cit. 


THE  PRECIPITATION  AND  WEIGHING  OF  ARGENTIC  BROMIDE.  39 

nitric  acid  to  prevent  colloidal  solution.  With  this  latter  faintly  acid 
liquid  the  precipitate  was  transferred  to  the  crucible.  The  silver  bromide 
was  dried  at  least  twelve  hours  in  the  filtering  crucible  at  130°  in  an  elec- 
tric oven/  and  weighed.  It  was  then  fused  in  a  porcelain  crucible,  in  a 
furnace  where  it  was  effectively  protected  from  flame  gases,  in  order  to 
determine  the  trace  of  moisture  always  retained.  The  wash-waters  and 
the  solution  obtained  on  neutralizing  the  ammoniacal  rinsings  of  the  flask 
gave  no  indication  of  the  presence  of  bromine  on  careful  testing  in  the 
nephelometer. 

In  using  the  Gooch-Munroe  crucible,  all  the  precautions  pointed  out 
in  the  preceding  paper  were  carefully  heeded.  A  perforated  plate  was 
always  placed  on  top  of  the  friable  sponge  to  prevent  rupture.  After  each 
determination  any  bromide  clinging  to  it  was  leached  out  with  potassic 
cyanide,  which  was  then  washed  away  with  nitric  acid,  and  finally  yery 
thoroughly  with  water.  The  solvent  action  of  the  cyanide  occasionally 
loosened  portions  of  the  spongy  film,  but  there  was  no  difficulty  in  repair- 
ing the  injury.  Upon  drying  in  the  electric  oven,  the  crucible  was  ready 
for  another  analysis. 

In  addition  to  the  loss  of  weight  on  fusion,  two  other  very  small  and 
somewhat  uncertain  corrections  were  applied  to  the  weight  of  the  argentic 
bromide,  but  because  they  were  of  about  the  same  magnitude  and  of 
opposite  sign,  their  effect  was  practically  negligible.  These  were  a  cor- 
rection for  platinum  corroded  from  the  boat  during  the  fusion  of  potassic 
bromide,  and  a  correction  for  argentic  bromide  dissolved  in  the  water  used 
for  washing.  Because  these  corrections  have  not  been  considered  in  most 
work  of  this  kind,  a  word  about  them  may  not  be  amiss. 

In  the  series  under  discussion  the  average  loss  of  the  boat  in  each  deter- 
mination was  less  than  0.00009  gram.  All  of  this  trace  of  platinum  may 
have  been  present  as  invisible  dust  in  the  solution,  and  thus  may  have  been 
weighed  with  the  argentic  bromide,  or  all  of  it  may  have  been  in  a  soluble 
form  and  may  have  remained  in  solution.  Because  of  this  uncertainty, 
a  compromise  was  made,  and  half  of  the  loss  of  weight  of  the  boat  (in  the 
mean  0.00004  gram)  was  subtracted  from  each  weight  of  argentic  bro- 
mide. As  the  average  total  weight  of  argentic  bromide  was  over  4  grams, 
this  compromise  could  not  have  introduced  an  error  as  great  as  1  part  in 
100,000  in  either  direction. 

According  to  Stas,^  argentic  bromide  is  wholly  insoluble  in  water,  but 
recent  experiments  show  that  in  the  flocculent  form  it  is  unquestionably 
soluble  to  a  slight  extent.^    As  in  the  case  of  the  chloride,  its  solubility  is 

^Richards,  Am.  Chem.  Joum.,  22,  45  (1899). 
2Stas,  Oeuvres,  I,  89. 

sBottger,  Z.  fiir  physik.  Chemie,  46,  602  (1903)  ;  Kohlrausch  und  Rose,  Zeit. 
phys.  Chem.,  12,  234  .(1893)  ;  also,  Richards,  Proc.  Am.  Acad.,  30,  385  (1894). 


40 


THE  ANALYSIS  OF  POTASSIC  BROMIDE. 


greatly  diminished  by  the  addition  of  an  excess  of  either  precipitant ;  but 
whether  or  not  any  is  dissolved  by  a  very  dilute  acid  solution  of  argentic 
nitrate,  such  as  that  used  in  washing,  it  is  practically  impossible  to  discover. 
It  is  not  unlikely,  however,  that  about  as  much  was  dissolved  by  this  solu- 
tion as  by  the  dilute  hydrobromic  acid  used  by  Baxter  in  his  admirable 
work  on  the  atomic  weight  of  bromine  ;^  and  in  this  solution  it  is  easy  to 
find  the  amount  of  dissolved  substance.  Baxter  found  as  a  matter  of  fact 
in  his  last  seven  most  exact  syntheses  an  average  of  0.00004  gram  of 
argentic  bromide  in  each  of  the  wash-waters  —  a  figure  which  was  added 
to  each  of  our  weights  of  argentic  bromide,  because  the  other  circumstances 
of  the  analysis  were  similar  in  the  two  cases.  On  the  average,  this  cor- 
rection exactly  eliminates  the  other ;  and  except  for  the  sake  of  complete- 
ness, they  might  both  have  been  wholly  neglected. 

Of  course  all  the  weighings  were  corrected  to  the  vacuum  standard,  by 
adding  0.000041  gram  to  every  apparent  gram  of  argentic  bromide  and 
0.00029  gram  to  every  apparent  gram  of  potassic  bromide,  as  calculated 
from  the  figures  6.473,  2.74,  and  8.30,  for  the  densities  of  argentic  bro- 
mide,^ potassic  bromide,  and  the  brass  weights  respectively.  All  the 
determinations  made  are  given  in  the  table  below. 

The  Ratio  of  Argentic  Bromide  to  Potassic  Bromide. 


Experiment 
number. 

Weight  of  KBr 
in  vacuum. 

Corrected  Weight 
of  AgBr  in  vacuum. 

Parts  of  KBr 

for  100.00  parts 

of  AgBr. 

Atomic  Weight  of 

Potassium 

Br  =  79.953. 

10 

11 

12 
16 

Total  average... 
Probable  error.. 

Grams. 

2. 19027 
4. 19705 
2. 06723 
2.58494 

Grams. 

3.45617 
6. 62285 
3. 26206 
4. 07889 

63.3728 
63. 3723 
63. 3719 
63. 3736 

39. 114 
39. 113 
39. 112 

39. 115 

11.0395 

17.41997 

63.3727 
0.0003 

39. 1135 
0.0004 

The  extreme  deviation  from  the  mean  corresponds  to  an  error  of  weigh- 
ing the  potassic  bromide  of  0.00004  gram,  a  reasonable  quantity.  The 
"probable  error"  indicates  that  there  is  but  little  chance  that  the  atomic 
weight  of  potassium  is  much  below  39.113  or  much  above  39.114,  if  con- 
stant chemical  errors  were  successfully  excluded.  Stas  found  in  a  single 
experiment  the  number  63.383  instead  of  63.373.^ 

With  unlimited  time  more  determinations  might  well  have  been  made, 
but  the  agreement  of  these  four  results  is  so  good  that  further  repetition 
seemed  to  be  not  very  urgent. 


^Baxter,  Journ.  Amer.  Chem.  Soc,  28,  1322  (1906). 
2Baxter  and  Hines,  Amer.  Chem.  Journ.,  31,  220  (1904). 
^Stas,  Untersuchungen  (Trans.  Aronstein  1867),  page  340. 


DETERMINATION  OF  THE  SILVER  NEEDED  FOR  PRECIPITATION.  41 

THE  DETERMINATION  OF  THE  SILVER  NEEDED  FOR  PRECIPITATION. 

The  silver  titration  method  of  Gay-Lussac,  as  developed  by  Pelouze, 
Mulder,  Stas,  and  more  especially  by  the  recent  work  at  Harvard,  yields 
very  consistent  and  accurate  results,  if  the  proper  conditions  are  carefully 
observed.  The  reading  of  the  end-point  is  rendered  easy  by  the  use  of 
the  nephelometer.^  In  the  case  of  the  somewhat  soluble  chloride,  various 
precautions  must  be  strictly  heeded ;  but  with  argentic  bromide,  which  is 
almost  insoluble,  the  matter  is  a  simpler  one,  and  a  very  slight  excess  of 
either  bromide  or  silver  can  be  easily  determined. 

The  method  used  in  the  present  case  is  easily  inferred  from  previous 
Harvard  work  of  the  same  kind.  From  the  weight  of  a  piece  of  the  purest 
silver,  the  equivalent  amount  of  potassic  bromide  was  calculated ;  slightly 
more  than  this  amount  of  substance  was  then  fused  as  before  in  a  plati- 
num-iridium  boat  placed  in  the  porcelain  tube  of  the  bottling  apparatus. 
From  the  weight  of  fused  bromide,  the  equivalent  quantity  of  silver  was 
calculated;  the  greater  part  of  the  difference  between  this  calculated 
weight  and  that  of  the  original  piece  of  silver  was  added  in  the  shape  of 
pure  silver  wire,^  and  any  final  difference  of  0.1  or  0.2  mg.  was  made  up 
with  a  dilute  solution  of  silver  nitrate.  The  silver  was  dissolved  in  nitric 
acid  and  the  nitrous  acid  expelled  as  usual,  and  the  solution  was  then 
diluted  to  about  tenth  normal. 

To  the  dilute  bromide  solution,  with  continual  agitation,  was  added  this 
dilute  argentic  solution;  the  total  volume  including  the  water  used  in 
rinsing  usually  amounted  to  about  1.5  liters.  After  shaking  steadily  for 
15  minutes,  and  occasionally  for  a  day,  the  mixture  was  allowed  to  settle 
during  another  day.  When  clear  above,  about  0.05  liter  of  the  aqueous 
solution  was  withdrawn  and  tested  in  the  nephelometer,  one  tube  being 
treated  with  an  excess  of  bromide,  the  other  with  an  excess  of  argentic 
nitrate.  If  any  difference  in  the  opalescence  was  noticeable  after  due  time 
had  been  allowed  for  the  very  faint  clouds  to  attain  their  maxima,  the 
slight  deficiency  was  made  up  in  the  flask  by  means  of  solutions  containing 
approximately  1  milligram  of  argentic  nitrate  or  of  potassic  bromide  per 
milliliter.  These  additions  were  continued  till  equality  in  the  opalescence 
was  attained.  From  the  sum  of  the  original  weights  and  subsequent  addi- 
tions, the  total  amounts  of  bromide  and  silver  were  obtained ;  from  these, 
reduced  to  vacuum,  the  ratio  was  calculated.  All  the  analyses  which  were 
performed  are  given  in  the  table,  excepting  No.  2,  which  was  rejected  for 
just  cause  before  it  was  finished.  Vacuum  corrections  of  +  0.00029  for 
every  gram  of  potassic  bromide  and  —  0.00003  for  every  apparent  gram 

iRichards  and  Wells,  Am.  Chem.  Journ.,  31,  235  (1904). 
2Richards  and  Parker,  Proc.  Am.  Acad.,  32,  60,  1896. 


42 


THE  ANALYSIS  OF  POTASSIC  BROMIDE. 


of  silver  were  applied.  Most  of  these  experiments  were  made  before 
those  given  in  the  preceding  table,  a  fact  which  may  account  for  the 
slightly  less  satisfactory  agreement  of  the  individual  results.  Because  the 
deviations  could  not  be  traced  to  any  definite  cause  of  disturbance,  they 
must  be  ascribed  to  accident. 


The  Ratio  of 

Potassic  Bromide  to  Silver. 

Experiment 

Weight  of  KBr 

Weight  of  Silver 

Parts  of  KBr 

Atomic  Weight  of 

number. 

in  vacuum. 

in  vacuum. 

corresponding  to 
100.00  parts  Ag. 

Potassium 
if  Br  =  79.953. 

Grams. 

Grams. 

1 

4. 33730 

3.93164 

110.318 

39. 113 

3 

4. 18763 

3. 79587 

110. 320 

39.115 

4 

4. 15849 

3. 76943 

110. 321 

39. 116 

5 

3. 67867 

3. 33450 

110.321 

39. 1 16 

6 

3. 60484 

3. 26776 

110.  315 

39.110 

7 

4.  78120 

4. 33387 

110. 322 

39. 118 

8 

5.67997 

5. 14860 

110. 321 

39. 116 

9 

6. 41587 

5.  81571 

110. 320 

39. 115 

13 

2.88134 

2. 61184 

110.318 

39. 113 

14 

3. 64383 

3. 30309 

110.316 

39.111 

15 

Total  average... 

3. 12757 

2. 83504 

110.318 

39. 113 

110.3190 

139. 1143 

Probable  error.. 

0. 0004 

0.0004 

J  The  average  39.1143  is  calculated  from  the  average  110.3190,  not  from  the  average  of  the  individual  values  of 
the  atomic  weights  in  the  column  above.    The  difference  is  of  course  very  slight. 

The  "probable  error"  is  as  small  as  before  because  of  the  greater 
number  of  determinations ;  and  the  mean  deviation  from  the  average  value 
is  only  one  in  the  last  decimal  place.  It  will  be  observed  that  these  results 
point  to  the  limits  39.113  and  39.115  as  the  extreme  values  between  which 
the  atomic  weight  of  potassium  must  fall,  in  essential  agreement  with  the 
previous  results. 

Marignac's  seven  experiments  on  this  ratio  give  values  ranging  from 
110.303  to  110.369,  while  Stas's  fourteen  results  ranged  from  110.332  to 
110.361. 

Obviously  the  results  furnish  a  means  of  calculating  the  atomic  weight 
of  bromine,  when  taken  in  connection  with  the  foregoing  series,  entirely 
independent  of  any  other  work.  Thus  Br  =  (110.319/63.3727  — 1.00000) 
107.93  =  79.954,  a  value  almost  identical  with  Baxter's  value,  79.953.  This 
is  excellent  proof  that  the  bromine  used  in  the  present  research  was  pure, 
and  that  the  occlusion  of  electrolytes  by  argentic  bromide  was  small. 


THE  ATOMIC    WEIGHT   OF   POTASSIUM.  43 

THE  ATOMIC  WEIGHT  OF  POTASSIUM. 

The  preceding  paper  and  the  present  one  together  yield  four  ratios 
determined  with  modem  precision,  which  together  fix  the  atomic  weight 
of  potassium  as  definitely  as  could  be  expected.  The  respective  values  are 
as  follows: 

From  the  ratio  of  argentic  to  potassic  chloride K  =  39.1134 

From  the  ratio  of  metallic  silver  to  potassic  chloride    ....  K  =  39.1 145 

From  the  ratio  of  argentic  to  potassic  bromide K  =  39. 11 35 

From  the  ratio  of  metallic  silver  to  potassic  bromide     ....  K=:  39. 11 43 


Average  atomic  weight  of  potassium,  if  Ag  =  107.930  .    .    .  K  =  39.1139 

These  figures  are  interesting  and  significant.  The  maximum  departure 
from  the  mean  is  only  1  part  in  70,000,  and  such  differences  as  exist  in  the 
figures  are  explicable.  It  is  likely  that  the  slightly  lower  value  given  by 
the  first  member  of  each  pair  of  series  was  due  to  a  trace  of  occlusion  of 
potassic  nitrate  by  each  of  the  precipitates  —  a  circumstance  which  can  not 
be  absolutely  prevented.  Therefore  the  higher  values,  averaging  39.1144, 
are  more  probable.  The  diflferences  are,  however,  wholly  negligible  at 
present. 

In  this  connection  it  is  interesting  to  note  that  Clarke  in  1897,  from  a 
miscellaneous  collection  of  partly  uncertain  results  obtained  by  others, 
decided  upon  the  almost  identical  value,  39.112,^  although  at  the  same  time 
from  similar  results  he  obtained  values  much  too  low  for  chlorine  and 
iodine,  somewhat  too  low  for  bromine,  and  much  too  high  for  sodium. 

The  very  close  mutual  agreement  of  the  new  results  obtained  from  two 
different  compounds  is  a  satisfactory  verification  of  the  relative  values 
yielded  by  the  recent  work  on  the  atomic  weights  of  chlorine  and  bromine.* 
The  value  of  chlorine  found  from  the  work  on  potassium  chloride  was 
35.475 ;  and  that  of  bromine  from  the  present  work  is  79.954.  The  ratio 
of  chlorine  to  bromine  is  thus  found  to  be  0.44369,  whereas  the  ratio  com- 
puted from  the  work  of  Richards  and  Wells  and  of  Baxter  is  0.44367. 

Against  such  an  accumulation  of  concordant  data  as  that  just  presented, 
the  older  figures  can  have  no  important  weight.  Whatever  may  have  been 
the  cause  of  the  irregularity  and  internal  inconsistency  of  Stas's  results 
with  potassic  chloride  and  bromide,  there  seems  to  be  little  reason  to 
doubt  that  the  outcome  of  the  present  investigation,  39.114,  really  repre- 
sents the  atomic  weight  of  potassium. 

It  is  needless  to  point  out  that  this  change  in  the  atomic  weight  of  potas- 
sium will  affect  many  other  atomic  weights. 

^Recalculations,  p.  57. 

2Richards  and  Wells,  loc.  cit.;  Richards,  Trans.  Amer.  Phil.  Soc,  43,  116  (1904)  ; 
Baxter,  loc.  cit. 


44  THE  ANALYSIS  OF  POTASSIC  BROMIDE. 

SUMMARY. 

This  investigation  upon  the  atomic  weight  of  potassium  presents,  among 
other  considerations,  the  following  additions  to  the  knowledge  of  the 
subject: 

(1)  The  problem  of  preparing  pure  potassic  bromide  was  solved  in 
two  ways. 

(2)  An  unusually  satisfactory  method  for  preparing  pure  potassium 
hydroxide  was  developed.  This  method  is  applicable  to  other  alkalies,  and 
will  be  described  elsewhere  in  greater  detail. 

(3)  The  ratio  of  silver  to  potassic  bromide  was  redetermined,  and 
found  to  be  100.000 :  110.319.  The  atomic  weight  of  potassiuiT?  was  thus 
found  to  be  39.1143,  if  silver  is  107.930  and  bromine  79.953. 

(4)  The  ratio  of  argentic  bromide  to  potassic  bromide  was  found  to  be 
100.000:  63.373.  This  determination  yielded  an  essentially  equal  value, 
K  =  39.1135. 

(5)  These  values  coniirm  in  a  striking  manner  the  simultaneously 
executed  work  upon  potassic  chloride,  and  unite  with  them  in  showing 
that  the  atomic  weight  of  potassium  is  39.114. 

(6)  By  thus  agreeing,  these  four  values  support  the  new  value  for  the 
atomic  weight  of  chlorine  in  relation  to  silver  and  bromine. 


Ill 

The  Quantitative  Synthesis  of  Argentic  Nitrate 

AND  THE  Atomic  Weights  of  Nitrogen 

AND  Silver 


By  Theodore  William  Richards  and  George  Shannon  Forbes 


Contributions  from  the  Chemical  Laboratory  of  Harvard  College 


THE  QUANTITATIVE  SYNTHESIS  OF  ARGENTIC  NITRATE,  AND  THE 
ATOMIC  WEIGHTS  OF  NITROGEN  AND  SILVER. 


INTRODUCTION. 

The  composition  of  argentic  nitrate  is  one  of  the  questionable  premises 
in  the  lively  argument  which  has  recently  taken  place  concerning  the 
atomic  weights  of  nitrogen  and  silver.^  Although  Stas's  syntheses  of  this 
salt  were  carried  out  on  a  large  scale,  and  far  more  carefully  than  those  of 
anyone  before  him,  several  points  concerning  the  details  of  the  work  were 
not  investigated  with  the  care  which  modern  physicochemical  knowledge 
demands.  Accordingly,  a  repetition  of  this  work  of  Stas's  seemed  to  be 
worth  the  trouble  involved;  and  the  following  pages  contain  a  brief 
account  of  nine  months'  thought  and  labor  upon  it. 

The  method  appears  at  first  sight  to  be  extremely  simple,  consisting 
merely  in  the  weighing  of  pure  silver,  the  dissolving  of  this  silver  in  nitric 
acid,  and  the  weighing  of  the  resulting  nitrate.  The  preparation  of  pure 
silver  having  been  already  solved,  the  great  difficulty  consisted  in  the 
procuring  of  satisfactory  evidence  that  the  nitrate  was  free  from  impurity, 
and  in  making  sure  that  none  of  the  silver  was  lost  during  the  process. 
The  main  emphasis  of  the  subsequent  discussion  will  therefore  be  laid  on 
these  points,  the  other  details  being  often  indicated  with  but  few  words, 
because  they  so  closely  resemble  the  details  of  previous  investigations 
carried  out  in  the  Chemical  Laboratory  of  Harvard  College. 

The  research  naturally  divided  itself  into  four  sections,  namely,  first, 
the  preparation  of  pure  materials;  second,  the  quantitative  synthesis; 
third,  the  determination  of  the  purity  of  the  product ;  and  fourth,  the  final 
result  and  its  relations  to  the  table  of  atomic  weights.  These  will  be  con- 
sidered in  order. 

PREPARATION  OF  PURE  MATERIALS. 

All  the  substances  used  in  the  research  were  purified  with  very  great 
care.     Nitric  acid  and  silver  were  the  two  most  important. 

Nitric  acid  was  supplied  by  two  firms.  Each  sample  was  warranted 
by  the  manufacturers  to  be  of  a  very  high  grade  of  purity.  Each  was 
redistilled  twice  just  before  adding  to  the  silver,  using  only  the  middle 

^See  especially  Guye,  Nouvelle  Rech.  s.  1.  Folds  Atom,  de  I'azote,  Soc.  Ch.,  Paris, 
1905.  The  reader  is  referred  for  a  convenient  resume  to  the  Report  of  the  Interna- 
tional Committee  on  Atomic  Weights,  Journ.  Am.  Chem.  Soc,  28,  1  (1906),  and 
many  other  places;  also  to  Clarke,  Journ.  Am.  Chem.  Soc,  28,  293  (1906);  Gray, 
Trans.  Chem.  Soc  (London),  89,  1173,  (1906). 

47 


48  THE  QUANTITAVE  SYNTHESIS  OF  ARGENTIC  NITRATE,  ETC. 

portion  of  each  distillate.  None  of  the  samples  left  a  trace  of  nonvolatile 
residue  on  evaporation.  No  difference  could  be  detected  in  the  results 
because  of  the  difference  of  source  of  the  acid,  nor  was  the  constancy 
destroyed  when  a  single  distillation  only  was  made.  (Experiment  11.) 
Hence  little  anxiety  was  felt  concerning  the  purity  of  this  material. 

Silver.  —  The  researches  of  Richards  and  Wells  have  shown  how  to 
prepare  silver  of  unquestioned  purity.  Preliminary  determinations  1,  2, 
3,  4,  5,  7,  and  8  were  made  on  a  sample  (A)  left  over  from  the  above- 
mentioned  research,  crystallized  fifteen  times  with  nitric  acid,  precipitated 
with  formate,  and  fused  on  purest  charcoal,  but  not  electrolyzed  or  fused 
in  hydrogen.  No.  9  was  made  with  a  sample  prepared  by  Professor 
Baxter  for  his  final  work  on  bromine.  It  had  been  through  the  chloride 
and  formate  treatment,  electrolyzed,  fused  in  hydrogen,  cut  with  a  saw, 
etched  with  nitric  acid,  boiled  with  water,  dried  at  dull  redness  in  vacuo, 
and  given  us  ready  to  weigh.  We  are  much  indebted  to  him  for  furnishing 
this  check  on  our  silver.  The  final  determinations  were  made  with  silver 
derived  from  several  preparations.  The  first  of  these  (C)  was  recrystal- 
lized  sixteen  times  as  nitrate  from  water  and  redistilled  nitric  acid,  and 
then  precipitated^  twice  in  succession  with  formate.  A  part  of  the  final 
formate  product  was  fused  on  the  best  lime  in  carefully  purified  hydrogen 
made  from  aluminum  and  sodic  hydroxide.  Sample  D  had  been  precipi- 
tated as  chloride,  reduced  with  best  alkaline  sugar,  washed  free  from 
chloride,  dissolved  in  nitric  acid,  filtered  and  crystallized  as  nitrate  in 
platinum  six  times  from  nitric  acid  distilled  in  a  platinum  condenser. 
Centrifugal  treatment  eliminated  the  mother  liquor.  The  last  crop  of 
crystals  was  precipitated  with  formate  in  a  silver  dish,  washed  free  from 
ammonia,  and  fused  in  a  cup  of  pure  lime.  Sample  F  was  obtained  by 
the  electrolysis  of  a  button  of  very  pure  silver  from  Colorado,  which  had 
been  fifteen  times  recrystallized  as  nitrate,  precipitated  with  formate,  and 
fused  on  lime.  Samples  C,  D,  and  F  were  all  cleaned  by  etching  and  then 
purified  by  electrolysis  through  a  nitrate  solution  made  from  some  of  the 
same  silver  and  the  purest  nitric  acid.  They  were  then  fused  separately, 
on  a  well-seasoned  lime  boat,  in  a  new  porcelain  tube,  in  an  atmosphere 
of  pure  electrolytic  hydrogen.  The  Hempel  stoppers  fitted  so  well  that 
the  pressure  could  easily  be  reduced  to  a  fraction  of  a  millimeter  by  a 
Geryk  oil-pump  when  desired.  The  heating  was  accomplished  by  a  large 
Heraeus  electric  furnace  which  fused  the  silver  without  overheating  any 
part  of  the  tube.  In  all  cases  the  initial  fusion  was  completed  in  hydrogen 
at  atmospheric  pressure,  but  in  half  the  fusions  the  tube  was  evacuated 
before  the  temperature  was  lowered.  No  spurting  or  boiling  could  be 
observed  through  the  glass  window  when  the  pressure  was  reduced,  and 


PREPARATION  OF  PURE  MATERIALS.  49 

the  silver  thus  prepared  gave  the  same  combining  weight  as  that  cooled 
under  a  full  atmosphere's  pressure  of  hydrogen.  Thus  the  conclusion  of 
Richards  and  Wells  and  of  Baxter  that  silver  can  not  dissolve  a  weighable 
amount  of  hydrogen  was  confirmed. 

The  buttons  thus  obtained  were  etched  to  remove  lime,  and  if  too  large 
to  go  into  the  flasks  used  for  the  synthesis,  were  cut  with  a  cold  chisel  or 
a  jeweler's  saw,  observing  all  the  precautions  recommended  by  Richards 
and  Wells.  The  fragments  freed  from  superficial  iron  were  washed  and 
dried,  sometimes  in  the  electric  oven  in  air  at  150°  for  an  hour,  sometimes 
in  a  vacuum  at  dull  redness,  and  sometimes  in  a  reduced  pressure  of  hydro- 
gen. Judging  from  the  constant  combining  weight,  all  these  methods 
were  equally  good. 

Water. —  The  water  was  distilled  first  with  alkaline  permanganate 
through  a  glass  condenser,  and  then,  after  the  addition  of  a  small  drop 
of  dilute  sulphuric  acid,  through  a  carefully  cleaned  condenser  of  block 
tin.  Needless  to  say,  dust  was  excluded  as  much  as  possible,  and  distilla- 
tions were  conducted  immediately  before  the  water  was  needed,  in  order 
to  avoid  absorption  of  gases  or  solution  of  solid  matter. 

Air. —  It  will  be  remembered  that  Stas  allowed  his  solutions  of  argentic 
nitrate  to  evaporate  very  slowly,  the  vapors  diffusing  out  through  the  neck 
of  his  flask  and  condensing  in  a  suitable  receptacle.  Under  these  condi- 
tions the  evaporation  required  seventy-two  hours  of  continuous  heating — 
an  unnecessarily  prolix  process.  To  hasten  the  escape  of  aqueous  vapor, 
it  was  resolved  to  maintain  a  gentle  current  of  pure  dry  air  throughout  the 
process.  The  air  for  this  purpose  was  delivered  from  a  water  pump, 
which  must  have  removed  some  of  the  original  impurities.  It  was  passed 
first  through  a  tall  Emmerling  tower  filled  with  beads  moistened  with 
concentrated  sulphuric  acid,  to  which  a  trace  of  potassic  bichromate  had 
been  added,  and  then  through  two  more  towers  filled  with  a  concentrated 
solution  of  pure  potash,  being  thus  freed  from  ammonia  and  from  acid 
gases.  Next  it  was  passed  through  a  tall  drying  tower  of  stick  potash,  to< 
a  hard  glass  tube  containing  platinized  asbestos  heated  to  dull  redness  by 
a  Bunsen  burner.  The  hot  platinum  was  intended  to  destroy  organic- 
matter.  Hence  it  passed  into  a  trap  designed  to  catch  asbestos  shreds,, 
and  finally,  the  air  was  dried  by  two  towers  of  broken  potash.  The  whole- 
purifying  train  was  put  together  without  rubber,  all  the  pieces  being- 
blown  together  except  the  hard  glass  tube,  which  was  connected  with  the- 
soft  glass  on  either  side  with  joints  so  well  ground  that  no  lubricant  wa& 
necessary.  The  towers  were  glass-stoppered.  Air  thus  purified  and  dried 
contains  nothing  to  injure  the  silver  nitrate  except  possibly  a  minute 
trace  of  ammonia,  which  might  have  come  out  of  the  potash  solutions. 


50  THE  QUANTITATIVE  SYNTHESIS  OF  ARGENTIC  NITRATE,  ETC. 

THE  SYNTHESIS   OF  ARGENTIC   NITRATE. 

The  materials  having  been  prepared,  the  next  step  was  to  combine  them. 
This  combination  must  take  place  in  a  vessel  wholly  free  from  a  sus- 
picion of  solubility  on  the  one  hand,  and  arranged  so  as  to  prevent  loss  of 
material  on  the  other  hand.  At  the  same  time,  it  was  important  to  be  able 
to  evaporate  and  to  weigh  the  argentic  nitrate  in  the  same  vessel.  After 
a  careful  consideration  of  the  somewhat  conflicting  claims  of  these  require- 
ments it  was  decided  to  carry  out  the  process  in  small  round-bottomed 
flasks  of  fused  quartz  of  from  0.03  to  0.04  liter  capacity;  with  necks 
11  cm.  in  length  and  6  to  9  mm.  in  internal  diameter.  Collars  of  platinum 
wire  provided  with  loops  permitted  the  suspension  of  these  flasks  from 
the  hook  of  the  Troemner  balance.  Before  weighing,  they  were  heated  in 
the  oven  to  be  presently  described  to  250°  for  at  least  half  an  hour,  a 
moderate  air-current  sweeping  through  them  all  the  time.  The  cover- 
ings were  then  lifted,  and  each  flask  was  removed  by  a  hook  of  platinum 
wire  sealed  into  a  glass  rod,  and  transferred  to  a  large  desiccator  pro- 
vided with  a  suitable  support.  Two  flasks  similar  in  weight  and  surface 
were  treated  in  the  same  way  and  kept  in  the  same  desiccator.  After 
two  hours  near  the  balance  the  first  flask  was  weighed  against  a  tare,  its 
weight  being  determined  by  substitution  of  the  first  by  the  second  flask 
and  a  few  small  weights.  Upon  removal  from  the  desiccator  both  flasks 
absorbed  moisture  from  the  air,  often  to  the  extent  of  0.0001  gram,  but 
they  soon  reached  constancy  with  respect  to  one  another,  which  was  all 
that  was  needed.  No  final  weighings  were  made  when  the  hygrometer 
in  the  weighing-room  stood  above  40,  and  under  these  circumstances 
flasks  could  be  weighed  on  successive  days  with  variations  not  exceeding 
0.00003  gram. 

A  new  Troemner  balance,  sensitive  to  0.01  mg.  was  available  for  the 
research.  The  brass  weights  were  carefully  standardized  at  the  begin- 
ning of  the  year,  and  again  upon  starting  the  final  series;  and  the  rider 
also  was  calibrated,  and  the  necessary  correction  employed.  For  cor- 
rection to  the  vacuum  standard  0.000132  was  added  to  each  apparent 
gram  of  argentic  nitrate,  and  0.000030  was  subtracted  from  each  apparent 
gram  of  silver.  This  was  on  the  assumption  that  the  two  specific  gravities 
concerned  were  respectively  4.35  and  10.49,  that  of  the  brass  weights 
being  8.3. 

The  silver  was  now  weighed  by  substitution,  and  carefully  pushed 
down  the  neck  of  the  flask,  which  was  held  meanwhile  in  a  horizontal 
position  to  avoid  breakage.  Next  the  whole  was  transferred  to  a  large, 
clean,  empty  desiccator,  whose  sides  had  been  moistened ;  and  the  flask  was 
laid  on  a  suitable  support  of  glass  and  platinum  tilted  45°  from  the  ver- 


THE  SYNTHESIS  OF  ARGENTIC  NITRATE. 


51 


tical.  A  sufficient  quantity  of  purest  nitric  acid  mixed  with  half  its 
volume  of  distilled  water  was  poured  in  from  the  platinum  crucible  into 
which  it  had  just  been  distilled.  The  cover  of  the  desiccator  was  replaced, 
but  its  stopper  was  first  removed,  and  covered  with  a  watchglass  and  a 
clean  beaker.  The  apparatus  was  now  kept  all  night  in  a  warm  place, 
between  40°  and  50°  C. ;  thus  the  silver  when  dissolved  remained  in  solu- 
tion, though  the  volume  was  restricted  to  2  cm.  of  solution  per  gram  of 
nitrate.  In  the  morning  the  process  was  found  to  be  complete.  After 
washing  down  the  neck  with  a  few  drops  of  freshly  distilled  water,  the 
flask  was  placed  in  the  oven  as  before.  The  desiccator  was  washed  with  a 
little  water,  the  washings  being  tested  for  silver  in  the  nephelometer ;  if 
any  had  been  found  it  might  have  been  feared  that  more  had  escaped,  but 
as  no  trace  was  ever  found,  it  was  inferred  that  the 
long  inclined  neck  had  caught  all  the  spray. 

Attention  is  now  called  to  the  apparatus  for  evapo- 
ration.^ The  main  feature  of  this  was  the  combined 
delivery  tube  for  the  air  current  and  hood  for  the 
protection  of  the  contents  of  the  flask.  This  hood  H 
with  its  ingress  and  egress  tubes,  /  and  E,  is  shown 
in  the  diagram  (figure  1).  The  dry  air  used  to 
sweep  out  the  aqueous  and  nitrous  vapors  entered 
from  the  purifying  train  at  G,  where  connection  was 
made  by  a  well-ground  joint;  passing  through  the 
tube  /,  it  escaped  into  the  body  of  the  flask  and  out 
through  its  neck.  A  side  tube,  E,  was  attached  to 
H  for  connection  with  a  water-pump.  The  flask 
rested  on  a  triangle  of  platinum  wire  attached  to  a 
glass  tripod,  and  the  whole  was  placed  in  a  1.5-liter 
beaker  which  served  as  the  oven.  A  sand-bath  en- 
abled this  to  be  kept  at  any  desired  temperature. 
The  cover  of  the  beaker  was  of  sheet  copper,  pierced 
with  three  holes,  for  the  tubes  /  and  E,  and  for  the 
thermometer.  As  no  nitrous  fumes  escaped  into 
the  oven,  this  cover  remained  intact.  In  the  prelimi- 
nary trials  the  extra  tube  Q  was  lacking,  the  tube 
/  being  extended  to  take  its  place.  The  part  which 
had  projected  into  the  body  of  the  flask  was  treated 
with  hot  dilute  nitric  acid  and  investigated  for  silver 


Fig.  1. — Apparalui  for  Evapo- 
radns  Aqueous  Solutioni. 
Section  about  half  actual  tize, 

/^  flask  of  fused  quartz. 

H,H,  protecting  hood,  with 
tube  /for  air  supply. 

Q,  quartz  tube  suspended 
by  platinum  wire,  serv- 
ing as  air-nozzle. 


^In  passing  it  might  be,  noted  that  one  of  us  used  a  somewhat  similar  arrange- 
ment for  evaporating  solutions  of  sodic  sulphate  fifteen  years  ago  (Proc.  Am. 
Acad.,  26,  258  (1891). 


62  THE  QUANTITATIVE  SYNTHESIS  OF  ARGENTIC  NITRATE^  ETC. 

in  the  nephelometer.  Occasionally,  some  was  found,  but  the  corrections 
required  were  not  large.  Nevertheless,  the  possibility  that  the  fused 
nitrate  had  attacked  the  glass  was  always  disturbing,  and  the  inconven- 
ience and  uncertainty  of  this  correction  made  its  elimination  desirable. 
Accordingly,  in  the  final  determinations,  a  section  of  quartz  tubing,  Q, 
3  mm.  in  diameter  and  15  mm.  long,  was  firmly  attached  to  a  platinum 
wire  and  lowered  till  it  barely  projected  into  the  body  of  the  flask.  This 
supporting  wire  hooked  into  the  collar,  so  that  the  quartz  tube  could  be 
removed  while  the  silver  was  dissolving.  During  the  evaporation,  how- 
ever, it  was  in  place,  and  the  end  of  the  glass  delivery  tube  was  shortened 
and  drawn  down  to  fit  into  it.  The  glass  tube  was  still  tested  as  before, 
but  no  silver  was  ever  recovered  from  it.  Hence  the  flask  and  the  quartz 
tube  must  have  retained  all  the  silver  originally  weighed  out. 

The  evaporation  of  the  liquid  was  easily  and  quickly  conducted  in  this 
apparatus.  If  the  flask  was  not  more  than  half  full  and  the  air  passed 
through  the  drying  train  no  faster  than  two  bubbles  per  second,  it  was 
possible  to  maintain  the  temperature  of  the  oven  at  125°  without  risk  of 
ebullition.  The  vapors  drawn  out  as  before  described  passed  first  through 
a  U-tube  filled  with  moist  glass  beads  and  just  sealed  with  water,  after- 
wards through  a  tower  of  caustic  alkali  to  take  out  nitric  acid  which  might 
attack  the  water-pump.  When  the  liquid  became  coated  with  a  film  of 
crystals  the  temperature  was  lowered  to  110° ;  bubbles  of  vapor  now 
formed  gradually  under  the  crystal  layer,  and  the  slight  spattering  caused 
by  their  rupture  could  be  seen  on  the  inside  walls  of  the  flask.  The  oven 
was  now  suddenly  cooled,  so  that  the  vapors  from  the  hot  liquid,  condens- 
ing on  the  walls  of  the  flask,  washed  down  the  crust  which  had  formed 
during  evaporation.  Upon  continuing  the  heating,  a  porous  crust  was 
soon  formed,  under  which  the  formation  of  steam  caused  no  spattering. 
Finally,  the  temperature  was  raised  gradually  while  the  last  trace  of 
liquid  was  disappearing.  The  solid  was  dried  for  a  quarter  of  an  hour 
at  150°  in  the  dark,  and  then  heated  to  230°^  to  insure  complete  fusion, 
not  only  of  the  main  portion,  but  also  of  any  portions  projected  on  the 
upper  walls  of  the  flask.  The  temperature  was  soon  lowered  to  210°, 
which  sufficed  to  keep  the  mass  in  fusion,  and  the  air  current  was  con- 
tinued for  an  hour.  Then,  in  subdued  light,  the  flask  was  lifted  out 
by  a  loop  of  platinum  wire  passed  through  its  collar,  placed  on  a  net- 
work of  platinum  wire,  and  tilted  in  all  directions  to  cause  the  nitrate  to 
solidify  in  a  thin  layer  on  the  walls.  Once  this  precaution  was  neglected 
and  the  flask  was  cracked  by  the  contraction  of  the  nitrate  on  cooling. 

^This  temperature  and  the  others  to  be  recorded  later  have  not  been  corrected  for 
the  exposed  steam.    They  do  not  indicate  the  exact  meeting-point  of  the  salt. 


THE  SYNTHESIS   OF   ARGENTIC    NITRATE. 


53 


When  the  nitrate  was  wholly  solid  but  still  hot,  the  flask  with  its  con- 
tents was  transferred  to  its  support  of  platinum  wire  in  a  large,  tight 
desiccator,  which  was  wrapped  in  a  black  cloth  and  left  near  the  bal- 
ance over  night.  Then  the  weight  of  the  flask  was  determined  as  before. 
Thus  in  the  entire  cycle  of  operations  nothing  but  platinum,  clean  glass, 
and  pure  air  touched  the  outside  of  the  flask. 

It  remains  to  be  proved  that  no  silver  was  carried  out  by  the  air  cur- 
rent. This  was  inferred  because  no  trace  of  silver  was  ever  found  in  the 
U-tube  through  which  all  the  vapors  passed;  but  more  definite  evidence 
was  obtained  by  redissolving  two  finished  determinations,  and  evaporat- 
ing them  again  with  the  customary  precautions. 


Determinttion 
number. 

AeNOs  once 
eraporated. 

AgNOs  twice 
evaporated. 

AeNOs  thrice 
eraporated. 

Total  gain. 

8 
15 

10.68933 
14.20114 

10. 68928 
14. 20122 

10. 68920 
14. 20127 

-0.00013 
+  0.00013 

Average  gain 

or  loss 

dbO.OOOOO 

The  first  residue  in  the  case  of  No.  15  was  slightly  darkened  by  pro- 
longed heating ;  still  no  fixed  tendency  toward  either  loss  or  gain  is  shown 
by  these    figures,  and  the  reliability  of  the  method  is  demonstrated. 

Two  preliminary  experiments  were  not  finished,  and  one  was  rejected 
because  of  known  error.  Six  preliminary  determinations  carried  out  in 
the  way  just  described  g^ve  results  varying  from  157.483  to  157.475 
parts  of  nitrate  from  100.000  parts  of  silver,  in  the  mean  157.479.  This 
is  very  nearly  the  same  as  the  final  series,  g^ven  later,  but  because 
of  sundry  irregfularities  these  determinations  are  individually  much  less 
trustworthy  than  the  latter.  One  of  the  preliminary  determinations  was 
made  with  a  piece  of  silver  kindly  given  us  by  Professor  Baxter  for 
the  sake  of  comparison,  and  for  which  we  are  much  indebted.  It  gavt 
a  result  perhaps  higher  than  the  average,  but  not  by  an  amount  greater 
than  the  limit  of  error  of  the  experimentation  at  that  time.  It  is  greatly 
to  be  regretted  that  this  piece  was  not  examined  after  every  detail  of  the 
process  had  been  perfected,  because  the  comparison  would  have  been  in- 
teresting, although  not  at  all  essential  for  the  completeness  of  our  work. 

When  it  appeared  impossible  to  improve  upon  the  details  of  the  experi- 
mentation, a  final  series  of  six  consecutive  determinations  was  carried 
out  with  all  possible  care.  In  each  case,  except  Nos.  14  and  15,  the  argen- 
tic nitrate  was  maintained  for  an  hour  in  a  fused  condition  while  dry  air 
swept  over  it.  In  No.  14  only  one-quarter  of  an  hour  was  allowed, 
whereas  in  No.  15  the  fusion  was  prolonged  for  three  hours.     Sample  D 


64 


THE  QUANTITATIVE  SYNTHESIS  OF  ARGENTIC  NITRATE,  ETC. 


of  silver  was  used  in  syntheses  10,  11,  and  12,  and  sample  F  in  the 
other  three. 

The  Synthesis  of  Argentic  Nitrate. 


No.  of 
eynthesia. 

Weight  of 
fused  silver 
(in  vacuum). 

Weight  of 

fused    argentic 

nitrate 

(in  vacuum). 

Weight  of  argen- 
tic nitrate  made 
from  100.000 
parts  of  silver. 

10 

11 

12 
13 
14 
15 

Average... 

Gramt. 

6.14837 
4. 60825 
4.97925 
9. 07101 
9. 13702 
9. 01782 

Gramt. 

9. 68249 

7. 25706 

7. 84131 

14.28503 

14.38903 

14. 20123 

157.481 
157.480 
157.480 
157.480 
157.481 
157.480 

1167.480 

^The  probable  error  of  this  average,  computed  from  the  results  when  car- 
ried to  the  next  decimal  place,  is  only  0.0001,  a  wholly  negligible  quantity. 
Hence  repetition  of  the  process  was  unnecessary. 


For  this  ratio  just  found  to  be  100.000 :157.480,  Stas  obtained  an  aver- 
age of  100.000:157.475  from  nine  determinations,  which  ranged  from 
157.463  to  157.488. 

Better  agreement  than  that  exhibited  by  the  above  table  could  hardly 
be  desired  or  expected,  as  the  greatest  deviation  corresponded  to  less 
than  0.1  mg.  in  the  weight  of  the  argentic  nitrate.  This  series  demon- 
strated that  it  is  not  necessary  to  use  very  large  amounts  of  material 
in  order  to  attain  a  very  high  order  of  precision,  if  only  the  details  of 
experimentation  are  fittingly  arranged. 

Satisfactory  as  this  series  of  results  appears  to  be,  it  is  by  no  means 
to  be  accepted  without  further  question  as  representing  the  true  weight 
of  argentic  nitrate  to  be  obtained  from  pure  silver.  Even  in  this  fused 
salt,  prepared  under  such  favorable  conditions,  several  impurities  might 
exist;  and  because  in  each  case  the  method  of  treatment  was  the  same, 
these  impurities  might  be  constant  in  amount  and  therefore  not  perceiv- 
able in  the  results.  Accordingly,  attention  was  now  directed  to  the  search 
for  these  impurities;  and  this  part  of  the  investigation  was  found  to  be 
the  most  arduous  and  time-consuming  part  of  it.  The  following  section 
discusses  this  matter. 


THE  PURITY  OF  THE  FUSED  ARGENTIC  NITRATE. 


65 


THE  PURITY  OF  THE  FUSED  ARGENTIC  NITRATE. 

The  first  impurity  for  which  search  was  made  was  air.  While  crystals 
obtained  from  solution,  although  always  containing  solvent,  may  be  sup- 
posed to  be  free  from  air,  the  fused  material  can  not  without  question 
be  assumed  to  be  free  from  this  impurity.  The  only  manner  of  approach- 
ing this  question  seems  to  be  to  fuse  the  salt  a  second  time  in  a  vacuum, 
in  order  to  detect  a  possible  loss  of  weight.  This  Stas  did  in  a  single  case, 
with  a  negative  result.  Nevertheless,  a  single  experiment,  even  of 
Stas's,  does  not  carry  with  it  much  weight;  therefore  the  test  deserves 
repetition. 

This  was  carried  out  in  the  present  case  without  great  difficulty  on 
five  of  the  preliminary  determinations.  The  flasks  containing  the  argen- 
tic nitrate  which  had  been  fused  for  an  hour  in  air  were  lowered  into 
mammoth  test-tubes  —  long  tubes  of  soft  glass  4.5  cm.  in  diameter  — 
sealed  at  the  lower  end  and  scrupulously  clean.  A  hood  was  placed  over 
the  mouth  of  the  flask  to  prevent  fragments  of  glass  from  entering,  and 
the  tube  was  drawn  down  in  two  large  converging  blast  flames.  A  glass 
tube  was  fused  on  and  connected  with  an  efficient  mechanical  hand 
pump;  several  exhaustions,  followed  by  admission  of  dry  air,  excluded 
all  moisture.  Finally,  the  system  was  reduced  to  2  mm.  and  sealed,  and 
heated  to  210°.  After  having  been  maintained  in  a  state  of  fusion  in  the 
dark  for  half  an  hour,  the  salt  was  cooled  in  the  usual  cautious  fashion, 
and  air  was  admitted  very  slowly.  The  flask  was  placed  in  a  desiccator 
over  night  and  weighed.  The  product  was  always  slightly  discolored, 
indicating  decomposition,  but  showed  no  serious  loss  in  weight. 

The  Eifect  of  Fusion  in  Vacuum. 


Determination 
number. 

Orieinal 
weight. 

Weight  after 
fusion. 

Change. 

1 

2 
3 
4 
9 

Grams. 

8.04489 
8. 75813 
8.54170 
9. 87850 
10. 76381 

Grams. 

8.04477 
8. 75810 
8. 54170 
9. 87837 
10. 76374 

—  0.00012 

—  0.00003 
±0.00000 

—  0.00013 

—  0.00007 

Forty-five  grams  of  salt  lost  in  all  0.00035  gram.  This  is  less  than  1 
part  in  100,000,  and  may  safely  be  referred  to  the  trace  of  decomposi- 
tion indicated  by  the  pale-brownish  coloration.  Not  only  is  it  reasonable 
to  suppose  that  but  little  air  is  dissolved  (for  according  to  the  law  of 
Henry  nearly  all  of  it  should  have  been  expelled  by  this  treatment)  but 
also  one  is  led  to  infer  that  very  little  water  remains  to  be  expelled.    This 


56 


THE  QUANTITATIVE  SYNTHESIS  OF  ARGENTIC  NITRATE^  ETC. 


matter  is  not,  however,  so  simply  settled ;  the  final  testing  of  it  cost  much 
labor. 

It  has  been  ordinarily  assumed  that  water  is  entirely  expelled  from  crys- 
tallized or  evaporated  salts  upon  fusion.  This  is  in  all  probability  the 
case  with  sodic  and  potassic  chlorides,  and  other  salts  which  fuse  at  high 
temperatures  and  show  little  affinity  for  water.  Whether  or  not  this  was 
the  case  with  argentic  nitrate,  which  at  210°  is  miscible  with  water  in  all 
proportions,  was  a  question  which  required  experimental  investigation. 

Besides  the  conservation  of  weight  on  fusion  in  a  vacuum  another  argu- 
ment may  be  adduced  to  show  that  most  of  the  water  has  been  driven  out. 
The  combining  weight  of  argentic  nitrate  seems  to  be  nearly  independ- 
ent of  the  time  of  its  fusion  in  a  current  of  dry  air,  as  is  shown  by  the 
following  table. 


Determination. 

Time  kept  in 
fusion. 

Weight  of  AgNOs 
formed  from 
100  parts  Ag. 

14 

9, 10, 11, 12, 13 

15 

15  minutes 

1  hour 

157.481 

157.480 
157.480 

3  hours 

It  should  be  stated  that  the  last  residue  was  slightly  discolored,  and 
increased  0.00008  gram  upon  a  subsequent  evaporation  with  water  and 
nitric  acid ;  hence  a  real  loss  of  weight  during  the  extra  period  of  fusion  is 
indicated;  but  this  was  probably  due  to  decomposition  of  the  salt  rather 
than  to  escape  of  water ;  therefore  the  table  bears  out  the  contention  stated 
above.  In  any  case  this  loss  amounts  to  much  less  than  a  unit  in  the  last 
decimal  place. 

Ordinarily,  in  the  past,  the  investigator  has  been  satisfied  with  such  an 
outcome  and  has  gone  no  further.  But  in  this  case  we  were  anxious  to 
leave  no  stone  unturned;  and  accordingly  a  drastic  method  of  treatment 
was  adopted,  which  permitted  no  trace  of  water  to  escape  being  weighed. 
This  was  to  decompose  completely  the  argentic  nitrate  by  heat,  and  to  pass 
the  gaseous  product  of  the  decomposition  through  a  weighed  tube  contain- 
ing phosphoric  oxide. 

A  hard  glass  combustion  tube  was  bent  and  drawn  out  in  the  manner 
shown  in  figure  2,  the  space  between  C  and  D  being  packed  with  glass 
wool.  Into  the  space  between  B  and  C  was  then  introduced  about  50 
grams  of  argentic  nitrate  which  had  been  crystallized  from  nitric  acid, 
barely  fused  in  porcelain,  cooled,  and  pounded  in  a  mortar.  The  tube 
was  heated  to  220°  in  an  air-bath  for  an  hour  in  a  stream  of  dry  air  passed 
in  through  A.  The  argentic  nitrate  should  now  be  in  a  condition  some- 
what comparable  to  that  of  the  quantitative  determination.     Finally,  a 


THE  PURITY  OF  THE  FUSED  ARGENTIC  NITRATE.  67 

weighed  pentoxide  tube  was  attached  at  D,  and  the  temperature  raised  to 
500°.  Yellow  oxides  of  nitrogen  came  over,  also  a  fine  gray  dust  caused 
presumably  by  the  bursting  of 
bubbles  of  decomposing  salt. 
After  an  hour  the  pentoxide 

tube    was    removed,    swept   out  p^.  2.-Tube  U  Decomooridon  of  Arctic  Ni.«te. 

with  air,  and  weighed  again.  ''^'  one-dghth  »ctuai  «ze. 

A  trace  of  the  gray  dust  had  been  carried  in,  and  suggested  a  slight 
gain  in  weight,  but  by  no  means  enough  to  account  for  the  great 
increase  of  weight,  0,012  gram.  This  was  finally  traced  to  the  adsorption 
of  nitrous  fumes  by  the  pentoxide,  2  mg.  being  lost  upon  passing  dry 
air  over  the  tube  for  an  hour,  and  the  rest  giving  a  very  strong  test  for 
nitrous  acid  after  being  dissolved  in  water. 

In  the  effort  to  eliminate  this  seriously  disturbing  effect,  recourse 
was  next  had  to  spirals  of  copper  gauze  heated  in  a  combustion  tube,  in 
order  to  abstract  the  oxygen  from  the  nitric  oxide,  leaving  only  nitrogen. 
It  is  well  known  that  metallic  copper  adsorbs  hydrogen ;  hence  the  spirals 
were  first  superficially  oxidized  in  a  stream  of  dry  air,  and  then  reduced 
by  pure  dry  carbon  monoxide.  This  was  generated  by  heating  the  purest 
oxalic  and  sulphuric  acids  of  commerce  together,  the  carbon  dioxide  being 
absorbed  by  a  generous  train  of  pure  concentrated  potash  solution,  and  the 
monoxide  dried  first  by  a  tower  of  concentrated  sulphuric  acid,  with 
beads,  and  finally  by  two  towers  of  pounded  potash.  The  resulting  cop- 
per should  be  above  the  suspicion  of  containing  any  considerable  amount 
of  hydrogen  or  moisture.  The  argentic  nitrate  was  treated  as  before, 
and  the  decomposition  was  continued  until  traces  of  yellow  fumes  were 
noted  coming  past  the  copper.  This  meant  that  nitric  oxide  had  been 
coming  over  for  some  time,  since  the  last  part  of  oxygen  is  harder  to 
■detach  than  the  first.  When  swept  out  with  air,  the  gas  in  the  pentoxide 
tube  turned  yellow,  but  it  was  not  then  supposed  that  much  nitric  oxide 
could  be  absorbed  by  the  pentoxide  in  so  short  a  time.  Nevertheless, 
nitrous  acid  was  found  upon  solution,  even  although  the  gain  in  weight 
was  only  a  third  as  great  as  before. 

Efforts  were  now  made  to  reduce  accidental  water  from  rubber,  etc. 
All  joints  were  made  to  overlap,  so  that  the  rubber  surfaces  exposed  to 
nitric  oxide  might  be  small,  and  large  asbestos  screens  kept  them  from 
"being  overheated  by  the  adjacent  furnaces.  Fresh  sublimed  pentoxide 
was  used  in  the  weighing-tube,  which  was  carefully  imitated  as  to  volume 
and  surface  by  a  similar  tube,  to  be  used  as  a  counterpoise  in  weighing; 
"both  were  opened  after  coming  to  the  temperature  of  the  balance-room, 
wiped  with  a  clean,  slightly  damp  cloth,  and  weighed  by  substitution.  A 
substantial  plug  of  glass  wool  in  the  decomposition  tube  strained  the  gases 


68  THE  QUANTITATIVE  SYNTHESIS  OF  ARGENTIC  NITRATE^  ETC. 

before  passing  over  the  copper ;  it  had  been  heated  in  the  dry  air  to  remove 
moisture. 

To  eliminate  the  possibility  of  water  in  the  copper  gauze  and  to  test  the 
apparatus,  a  blank  run  was  made,  removing  the  decomposition  tube,  and 
oxidizing  the  copper  with  dry  air.  Here  the  pentoxide  tube  lost  0.0004 
gram,  a  result  which  indicated  no  error  in  that  part  of  the  process. 

It  is  not  worth  while  to  record  the  individual  experiments  by  which  the 
gradual  improvements  in  apparatus  and  manipulation  were  tested,  tire- 
some and  time-consuming  as  these  were  in  execution.  By  the  time  seven 
experiments  had  been  performed,  it  was  clear  that  the  fused  argentic  nitrate 
could  not  contain  over  0.004  per  cent  of  water.  Even  of  this  small  amount 
a  part  was  undoubtedly  due  to  nitric  peroxide ;  for  the  copper  gauze  was 
not  enough  to  effect  a  complete  decomposition  of  the  gases.  Passing  car- 
bon monoxide  through  the  apparatus  while  the  decomposition  was  in  prog- 
ress lengthened  the  effective  life  of  the  reduced  copper,  but  introduced 
other  complications;  and  the  addition  of  a  second  tube  of  hot  gauze,  a 
meter  long,  was  still  inadequate. 

The  next  step  was  to  secure  a  copper  surface  so  large  that  all  the  nitro- 
gen oxide  resulting  from  the  complete  decomposition  of  50  grams  of 
argentic  nitrate  could  be  removed,  and  this  without  increasing  the  compli- 
cations of  the  apparatus.  Powdered  copper  oxide  looked  promising  as  a 
source  of  this  metal,  but  it  must  be  made  from  pure  materials  to  avoid 
the  presence  of  detrimental  impurities.  Pure  electrolytic  copper  (Merck) 
was  dissolved  in  nitric  acid,  evaporated  to  dryness  in  porcelain,  and  very 
gradually  ignited,  with  constant  stirring,  to  powdery  oxide.  A  hard  glass 
tube  30  cm.  long  and  3  cm.  internal  diameter  was  used  to  contain  it ;  the 
ends  were  drawn  down  conveniently  and  the  oxide  was  reduced  at  a  very 
moderate  heat  with  carbon  monoxide,  prepared  as  usual,  except  that  it 
was  finally  dried  with  pentoxide.  Nearly  300  grams  of  copper  oxide  were 
present ;  therefore,  the  process  was  very  tedious,  since  the  gas  had  to  pass 
through  its  purifying  train  at  a  moderate  rate  of  speed.  With  this  large 
mass  of  finely  divided  copper  the  glass  wool  (from  C  to  D  in  the  dia- 
gram) was  no  longer  necessary,  therefore  it  was  no  longer  used.  No  trace 
of  powdered  nitrate  ever  appeared  beyond  the  copper. 

A  clean  pentoxide  tube,  with  glass  stopcocks,  was  refilled  with  resub- 
limed  pentoxide  and  glass  wool ;  dry  air  was  passed  through  for  ten  min- 
utes to  assure  constancy.  A  duplicate  tube  was  also  prepared  for  a  coun- 
terpoise, and  extraordinary  precautions  were  taken  in  protecting  and 
weighing  these  tubes.  As  another  precaution  it  seemed  also  expedient  to 
prepare  argentic  nitrate  nearly  as  pure  as  that  used  in  the  determinations 
for  fear  of  hygroscopic  nitrates,  silicic  acid,  and  ammonium  salts,  which 
would  yield  water  when  decomposed.    A  moderately  pure  salt  was  recrys- 


THE  PURITY  OF  THE  FUSED  ARGENTIC  NITRATE. 


69 


tallized  from  nitric  acid,  filtered,  precipitated  with  formate,  washed  free 
from  ammonium  salts,  and  fused  on  lime  in  the  blast.  The  buttons  were 
etched  and  dissolved  in  freshly  distilled  nitric  acid  in  Jena  glass,  and 
the  salt  was  crystallized  with  ample  protection  against  ammonia,  dried 
centrifugally  and  kept  in  a  clean,  tight  desiccator.  There  was  no  ammonia 
in  this  product.    Before  using  it  was  fused  in  platinum. 

In  the  first  determination  of  this  series  the  argentic  nitrate  was  heated 
too  hot  (520°)  and  the  gases  came  over  so  fast  that  the  reduced  copper 
was  only  superficially  attacked ;  even  so  it  was  vastly  more  efficient  than  the 
gauze.  Because,  however,  some  nitric  peroxide  still  escaped,  this  experi- 
ment also  had  to  be  rejected,  a  part  of  the  0.004  per  cent  impurity  which 
it  indicated  being  undoubtedly  due  to  nitrous  fumes. 

The  apparatus  was  now  improved  by  sealing  off  the  end  (A  )  of  the  tube 
after  the  argentic  nitrate  had  been  fused  for  a  long  time  in  a  current  of  dry 
air.  Moreover,  the  last  trace  of  rubber  exposed  to  nitrous  fumes  was 
eliminated  by  providing  an  accurately  ground  joint  between  the  decom- 
position tube  and  the  copper  oxide.  The  process  was  conducted  so  slowly 
at  490°  that  the  argentic  nitrate  could  be  entirely  decomposed  to  pure 
spongy  silver  without  exhausting  all  the  copper.  Care  was  taken  to  have 
the  very  pure  argentic  nitrate  in  a  state  comparable  to  that  used  in  the 
determinations.  The  salt  after  its  preliminary  treatment  in  platinum  still 
contained  a  few  crystals  of  unfused  nitrate ;  accordingly  in  the  tube  it  was 
heated  evenly  at  220°  with  shaking  to  dislodge  steam  bubbles,  exactly  as 
the  determinations  had  been.  Under  these  conditions  reliable  results 
were  obtained.  In  No.  25,  a  second  weighed  pentoxide  tube  placed  in 
tandem  with  the  first  showed  no  gain  in  weight;  hence  the  first  tube 
caught  all  the  water  vapor.  The  remaining  nitrogen  was  driven  out  by 
pure  dry  air  before  weighing.  That  water  vapor  was  actually  present 
was  shown  by  the  "melting"  of  phosphoric  pentoxide  in  the  first  tube. 

Water  Obtained  by  Decomposition  of  Argentic  Nitrate. 


Experiment 
number. 

Weieht  of 
AgNOs. 

Gain  in  weight 
of  drying  tubes. 

Percentaee  of 
water  found. 

25 
26 
27 

Average... 

Gram 

53 

52 
44 

Gram 

0.0015 
0.0011 
0.0014 

Per  cent 
0.0028 

0. 0020 
0. 0033 

50 

0.0013 

0.0027 

Thus  it  is  clear  that  less  than  0.003  per  cent  of  water  was  held  by  the 
argentic  nitrate  after  fusion.  There  was  only  one  other  source  from 
which  this  milligram  or  so  of  water  might  have  come,  namely,  from  the 


*60  THE  QUANTITATIVE  SYNTHESIS  OF  ARGENTIC  NITRATE,  ETC. 

copper  used  for  the  reduction  of  the  nitrous  fumes.  Less  than  0.0003 
gram  of  hydrogen  retained  by  300  grams  of  copper,  or  less  than  one  part 
in  a  million,  would  have  been  enough  to  cause  this  effect,  hence  it  seemed 
■rash  to  assume  that  the  water  did  not  come  from  this  source.  As  much 
hydrogen  as  this  might  have  come  from  a  trace  of  moisture  in  the  hun- 
dred liters  of  carbon  monoxide  used  for  the  reduction  of  the  copper. 

The  question  thus  raised  was  capable  of  being  investigated.  For  this 
purpose  the  mass  of  copper,  reduced  just  as  it  would  have  been  for  one  of 
the  preceding  experiments,  was  twice  oxidized  by  a  very  large  volume  of 
pure  ignited  air  dried  with  the  usual  potash  towers  and  finally  with  phos- 
phoric pentoxide.  This  air,  having  been  already  passed  over  red-hot 
cupric  oxide  in  a  hard  glass  tube  and  thoroughly  dried  by  pentoxide,  could 
hardly  bring  with  it  any  water  which  could  be  taken  up  by  the  following 
tube  of  the  same  material.  On  one  of  these  occasions,  0.0009  gram  of 
water  was  found,  and  on  another  0.0007  gram.  Possibly  some  of  this  may 
have  come  from  the  atmosphere  during  the  manipulation  of  the  tubes ;  but 
a  similar  amount  must  be  supposed  to  have  been  taken  during  each  of  the 
previous  determinations.  Hence  it  seems  to  be  permissible  to  subtract 
the  average  0.0008  gram  from  the  average  result  of  experiments  25,  26, 
and  27,  namely,  0.0013  gram.  This  leaves  only  0.0005  gram  as  the  maxi- 
mum amount  of  water  held  by  50  grams  of  carefully  fused  argentic  nitrate, 
introducing  an  error  of  only  1  part  in  100,000.  As  the  average  of 
the  final  series  led  to  a  value  a  trifle  over  157.480  grams  as  the  weight  of 
nitrate  obtainable  from  100.000  grams  of  silver,  the  correction  to  be  applied 
for  water  reduces  the  result  only  to  157.479.  Possibly  not  even  as  much 
water  as  this  was  present  in  the  quantitatively  made  argentic  nitrate, 
because  the  relative  surface  exposed  for  drying  was  not  so  great  in  the 
long,  narrow  tube  just  used  as  it  was  in  the  quartz  flask  employed  for 
-^experiments  10  to  15. 

This  unimportant  change  in  the  synthetic  result  from  157.480  to  157.479 
was  the  only  apparent  outcome  of  these  tedious  and  often  exasperating 
experiments  on  the  decomposition  of  the  nitrate ;  but  in  reality  more  was 
shown  by  them.  The  experience  furnished  to  the  experimenters  a  strik- 
ing example  of  the  essential  importance  of  taking  as  much  care  in  deter- 
mining a  small  correction  as  in  determining  the  quantity  to  be  corrected. 
It  moreover  confirmed  the  impression  that  except  in  cases  like  the  chloride 
of  zinc,  when  water  acts  chemically  upon  the  substance,  little  or  no  water 
is  retained  by  most  fused  salts.  If  argentic  nitrate,  which  at  200°  is  misci- 
ble  with  water  in  all  proportions,  sets  free  so  nearly  all  that  it  possesses 
in  a  current  of  dry  air  at  this  temperature,  it  is  mucih  more  likely  that 
other  salts,  fusing  at  a  higher  temperature  and  possessing  a  less  attraction 
for  water,  should  be  free  from  it  after  fusion.  This  is  a  reassuring  con- 
viction, well  worth  the  trouble  spent  upon  the  point. 


THE  PURITY  OF  THE  FUSED  ARGENTIC  NITRATE.  61 

Attention  was  now  directed  to  other  foreign  substances  besides  water 
and  air  which  might  be  present  in  the  synthetic  argentic  nitrate.  Further 
careful  investigation  was  needed  to  show  that  all  the  silver  was  combined 
as  nitrate  and  that  no  other  impurities  were  concealed  in  the  salt.  The 
pure  pearly  whiteness  of  the  cold  fused  material  precluded  the  possibility 
of  there  having  been  a  trace  of  reduction  to  metallic  silver.  The  salt  was 
soluble  in  pure  water  without  residue  or  turbidity,  and  maintained  its 
clearness  when  diluted  to  half  or  quarter  normal ;  hence  oxide  and  halogen 
salts  must  have  been  absent.  The  only  other  likely  impurity  seemed  to  be 
ammonium  salts,  nitrite,  and  excess  of  nitric  acid.  Of  these  possibilities, 
Stas  thought  only  of  the  likelihood  of  the  presence  of  free  acid ;  but  each 
was  carefully  considered  in  the  present  work. 

It  is  known  that  copper  dissolving  in  nitric  acid  forms  perceptible  quan- 
tities of  ammonic  nitrate.  Apparently,  no  similar  reaction  has  been  noted 
with  silver,  and  from  an  electrochemical  standpoint  it  seemed  unlikely; 
moreover,  ammonic  nitrate  if  present  would  probably  be  for  the  most  part 
decomposed  by  the  hour's  fusion  of  the  silver  salt.  Nevertheless,  thor- 
oughness demanded  evidence  on  this  point.  Nessler  solution  is  the  most 
convenient  for  this  test,  but  silver  must  first  be  precipitated  from  solution 
in  the  dark.    Light  generates  chlorine  which  interferes  with  the  test. 

In  order  to  prepare  for  this  search  for  ammonia,  the  synthetic  nitrate 
of  silver  resulting  from  several  experiments  was  shaken  with  a  slight  ex- 
cess of  standard  pure  sodic  chloride  until  the  supernatant  liquid  was  clear ; 
the  latter  was  decanted,  made  up  to  50  cc,  and  placed  in  a  comparison 
cylinder.    The  sodic  chloride  gave  no  test  with  Nessler  solution. 

The  cylinder  full  of  liquid  prepared  from  the  argentic  nitrate  was  finally 
tested  by  comparison  with  other  cylinders  containing  known  traces  of  am- 
monic nitrate  mixed  with  sodic  nitrate  in  concentration  equal  to  that  exist- 
ing in  the  actual  determination.  The  sodic  nitrate  had  been  several  times 
recrystallized  with  centrifugal  draining,  and  was  shown  to  be  free  from 
ammonia.  In  this  way  the  following  results  were  attained :  Synthesis  12 
was  found  to  contain  0.05  mg.  of  ammonic  nitrate,  synthesis  13,  0.06  mg., 
and  synthesis  14,  0.05  mg. ;  in  all,  36.5  grams  of  argentic  nitrate  were 
proved  to  contain  no  more  than  0.00016  grami  of  ammonic  nitrate  —  less 
than  one  two-thousandth  of  1  per  cent.  The  source  of  this  ammonia  was 
not  determined ;  it  might  have  come  from  the  reduction  of  nitric  acid  or 
from  the  large  volume  of  air  used  in  evaporating  the  argentic  nitrate. 
The  only  possible  place  where  it  could  have  entered  with  the  reagents 
used  in  the  test  was  in  the  sodic  chloride  solution ;  this  was  indeed  tested, 
with  negative  results,  but  Nessler's  reagent  is  not  very  sensitive  in  the 
presence  of  chloride,  and  a  trace  of  ammonia  might  have  eluded  detection. 
If  the  amount  found  had  been  larger,  more  time  would  have  been  spent 
upon  the  matter,  and  a  sample  of  the  argentic  nitrate  often  recrystallized 


"62  THE  QUANTITATIVE  SYNTHESIS  OF  ARGENTIC  NITRATE,  ETC. 

in  an  atmosphere  free  from  ammonia  would  have  been  used  in  a  blank 
-experiment  for  comparison;  but  with  such  a  small  trace  of  impurity  this 
was  not  worth  while. 

In  summing  up  the  results  of  the  test  for  ammonia,  the  amount  should 
be  calculated  for  the  quantity  of  argentic  nitrate  produced  from  100.000 
^rams  of  silver.  Thus  it  is  found  that  157.480  grams  of  fused  argentic 
nitrate  could  not  have  contained  over  0.0007  gram  of  ammonic  nitrate. 

The  next  impurity  to  be  studied,  nitrous  acid,  was  sought  by  means  of 
sulphanilic  acid  and  naphthylamine  hydrochloride.  These  were  proved 
with  a  standard  nitrite  solution  of  which  1  liter  contained  0.1  gram  of 
nitrogen.  To  insure  parallel  conditions  in  testing  by  comparison,  pure  sil- 
ver nitrate  was  prepared  with  some  care.  It  was  thrice  recrystallized 
from  nitric  acid  with  centrifugal  treatment,  fused  in  porcelain,  then  recrys- 
tallized three  times  more  from  the  purest  water  in  platinum,  with  centrif- 
ugal draining  as  before.  The  specimens  of  fused  argentic  nitrate  result- 
ing from  quantitative  determinations  were  dissolved  in  known  amounts 
of  water,  and  comparison  solutions  of  equal  concentration  prepared.  The 
crystallized  silver  nitrate  gave  no  trace  of  pink  color  with  the  reagents. 
A  fresh  solution  to  which  0.1  cc.  of  nitrite  solution  (0.01  mg.)  had  been 
added  gave  a  pronounced  color,  while  determinations  2,  3,  and  11  all 
gave  a  much  fainter,  barely  visible  color ;  hence  it  was  permissible  to  con- 
clude that  the  loss  of  oxygen  from  the  nitrate  on  fusion  could  not  have 
exceeded  0.000005  gram,  a  wholly  negligible  quantity. 

In  seeking  for  the  next  impurity,  free  nitric  acid,  Stas  tested  aqueous 
solutions  of  his  fused  silver  nitrate  with  "tournesol"  and  found  them  alka- 
line rather  than  acid.  This  alkali  may  have  come  from  his  glass  vessels. 
We  used  methyl  orange,  a  more  sensitive  indicator,  and  noted  that  the 
purest  crystallized  neutral  specimens  and  the  fused  salt  alike  gave  equally 
pronounced  acid  reactions  in  solution.  The  color  was  not  modified  by  the 
addition  of  half  a  milligram  of  nitric  acid ;  therefore  the  equality  in  color 
■does  not  disprove  the  presence  of  free  acid  in  our  fused  product.  Dilute 
sodic  hydroxide  freed  from  carbonate  by  baric  hydroxide  threw  down  a 
precipitate  without  changing  the  pink  color.  Hence  the  indicator  was  dis- 
carded and  the  investigation  continued  with  the  nephelometer. 

The  test  for  acid  with  the  help  of  this  instrument  was  conducted  by  add- 
ing a  very  dilute  standard  caustic  alkaline  solution  to  the  solution  to  be 
tested  and  by  observing  if  this  addition  caused  a  permanent  cloud. 

PreHminary  trials  were  necessary  in  the  first  place  in  order  to  determine 
the  conditions  best  suited  for  accuracy.  It  was  found  that  a  concentration 
of  nitrate  no  stronger  than  4  grams  in  0.025  liter  was  best  adapted  to  this 
work,  as  argentic  hydroxide  is  distinctly  soluble  in  double  this  concentra- 
tion of  argentic  nitrate.    On  the  other  hand,  argentic  hydroxide  precipi- 


THE  PURITY  OF  THE  FUSED  ARGENTIC  NITRATE.  63 

tated  by  somewhat  concentrated  alkali  does  not  quickly  redissolve  in  an 
equivalent  quantity  of  nitric  acid,  mixed  with  the  dilute  argentic  solution. 
It  was  found  that  even  the  precipitate  caused  by  0.1  ml.  of  a  two-hun- 
dredth normal  solution  may  produce  a  fairly  permanent  cloud  in  spite  of 
the  presence  of  an  excess  of  0.00003  gram  of  nitric  acid,  enough  to  dis- 
solve it.  Still  more  dilute  solutions  behaved  much  more  satisfactorily, 
however.  A  milliliter  of  a  two-thousandth  normal  caustic  solution  (equiv- 
alent to  0.00003  gram  of  nitric  acid)  added  with  constant  stirring  to  the 
pure  standard  argentic  nitrate  formed  a  cloud  easily  seen  in  the  neph- 
elometer.  In  case  0.00003  gram  of  nitric  acid  had  been  introduced 
before  adding  alkali,  no  precipitate  was  formed  under  the  same  conditions. 
Hence  under  these  circumstances  the  test  attained  a  degree  of  sensitive- 
ness suited  to  the  case  in  hand. 

In  this  way  it  was  found  that  the  argentic  nitrate  remaining  from  syn- 
thesis 11  gave  a  distinct  opalescence  with  the  addition  of  0.00002  gram  of 
sodic  hydroxide,  and  an  obvious  cloud  with  0.00003  gram.  Hence  it  was 
concluded  that  argentic  nitrate  fused  in  a  stream  of  pure  air  for  one  hour 
contains  no  weighable  excess  of  nitric  acid. 

The  various  suspected  impurities  having  thus  been  duly  sought,  it  is 
instructive  and  interesting  to  tabulate  the  results.     These  are  as  follows : 

Grams 

Weight  of  fused  AgNOs  from  100,000  grams  Ag 157.480 

Correction  for  weight  of — 

Dissolved  air 0.000 

Retained  water — 0.0016 

Retained  ammonic  nitrate — 0.0007 

Nitrite 0.000 

Free  acid 0.000 

Corrected  weight  of  argentic  nitrate  obtainable  from 

100.000  grams  of  pure  silver 157.478 

.  As  the  subtractive  corrections  given  in  the  above  table  are  maximum 
values,  the  weight  of  argentic  nitrate  in  question  can  hardly  be  lower  than 
this  value,  157.478.  On  the  other  hand,  it  can  hardly  be  higher  than  the 
uncorrected  value,  157.480.  Thus  two  limits  are  set,  very  near  together, 
between  which  the  true  value  must  lie.  Obviously,  for  the  present  one 
can  not  go  very  far  astray  in  accepting  the  mean  value,  157.479 ;  and  this 
value  will  be  used  in  the  following  discussion. 


64  THE  QUANTITATIVE  SYNTHESIS  OF  ARGENTIC  NITRATE,  ETC. 


THE  FINAL  RESULT  AND  ITS  RELATIONS  TO  THE  VALUES  OF  ATOMIC 
WEIGHTS  OF  NITROGEN  AND  SILVER. 

The  conclusion  reached  by  the  foregoing  series  of  experiments  is  that 
100.000  parts  of  silver  yield  very  nearly  157.479  parts  of  argentic  nitrate. 
Among  other  experimenters,  Penny  found  157.442;  Marignac  found 
157.424;  Stas  found  157.474  in  one  series  of  seven  determinations  and 
157.486  in  another  of  two,  and  Hardin  found  157.484.  From  these  older 
figures,  by  an  interesting  coincidence  Clarke  calculated  exactly  the  value 
found  in  the  present  research.^  This  coincidence  is  to  be  attributed  not 
so  much  to  the  efficacy  of  the  method  of  calculation  as  to  the  fact  that  in 
this  case  the  impurities  in  the  silver  happened  to  balance  exactly  the  im- 
purities in  the  argentic  nitrate.  At  first  sight  it  is  incomprehensible  how 
Stas,  working  with  slightly  impure  silver,  could  have  obtained  any  results 
higher  than  the  true  value ;  but  it  must  be  remembered  that  his  nitrate  was 
fused  in  a  glass  vessel,  which  must  have  been  attacked  by  the  strongly 
acid  nitrate  at  220°.  Constancy  in  the  weight  of  the  vessel  would  be  no 
evidence  of  absence  of  action,  for  silver  might  partly  take  in  the  glass  the 
place  of  the  sodium  taken  from  it.  Thus  a  gain  in  weight  due  to  sodic 
nitrate  might  be  accompanied  with  no  considerable  loss  of  weight  of  the 
vessel.  In  favor  of  this  hypothesis  is  the  fact  that  Stas's  results  steadily 
decreased  in  each  series,  as  he  proceeded  with  the  work ;  the  flask  seems  to 
have  been  less  and  less  susceptible  to  attack,  as  is  reasonable.  If  the  last 
two  determinations  of  the  first  series  are  taken  as  the  most  nearly  free  from 
this  cause  of  error,  the  number  157.466  obtained  from  them  should  furnish, 
by  comparison  with  our  number,  some  clue  to  the  amount  of  gaseous  im- 
purity in  Stas's  silver.  Thus  it  appears  that  his  silver  must  have  contained 
nearly  0.01  per  cent  of  impurity  —  probably  more  if  it  is  considered  that 
the  flask  was  still  not  wholly  resistant  —  a  conclusion  not  very  different 
from  that  reached  by  Richards  and  Wells. 

Speculations  of  this  kind  concerning  older  work  are  rather  a  thankless 
task,  however ;  there  is  usually  too  much  that  is  doubtful  to  allow  them  to 
serve  a  very  valuable  purpose.  The  only  object  in  pursuing  the  review  at 
all  is  in  order  to  assure  one's  self  that  no  real  inconsistency  exists  in  the 
data. 

^Recalculation  of  the  Atomic  Weights  (1897),  p.  64. 


THE  FINAL  RESULT  AND  SUMMARY.  65 

Attention  is  now  directed  to  a  more  important  matter,  namely,  the  effect 
of  the  new  experimental  result  upon  the  table  of  atomic  weights.  This  is 
quickly  stated:  The  newly  advocated  low  atomic  weight  of  nitrogen  is 
incompatible  with  the  ratio  100.000 :  157.479,  if  silver  is  taken  as  107.930. 
If  on  the  other  hand,  the  new  atomic  weight  of  nitrogen  is  the  true  one, 
silver  must  have  a  much  lower  value  than  this. 

The  exact  figures  are  given  in  the  following  table : 

The  Atomic  Weight  of  Nitrogen. 

If  Ag  =  107.930,  AgNOs  =  169.967  and  N  =  14.037 

If  Ag  =  107.890,  AgN08  =  169.904  and  N  =  14.014 

If  Ag=  107.883,  AgN03  =  169.893  and  N=  14.010 

If  Ag  =  107.880,  AgNOs  =  169.888  and  N  =  14.008 

This  series  of  conditional  statements  contains  in  a  nutshell  the  result  of 
the  present  investigation.  In  order  to  decide  between  the  alternatives, 
other  compounds  must  be  further  studied,  especially  the  chlorates  and  the 
ammonium  salts.  Investigations  in  both  of  these  directions  have  already 
been  begun  in  the  Chemical  Laboratory  of  Harvard  College. 

SUMMARY. 

Argentic  nitrate  was  made  from  pure  silver,  and  the  gain  in  weight  was 
carefully  noted. 

In  the  course  of  the  work,  a  new  and  convenient  apparatus  for  quantita- 
tive evaporation  was  devised.    Quartz  flasks  were  used  as  a  part  of  it. 

The  argentic  nitrate  was  fused  until  constant  in  weight;  it  was  care- 
fully tested  for  dissolved  air,  retained  water  and  ammonia,  and  nitric  and 
nitrous  acids.  Only  the  second  and  third  of  these  impurities  could  be 
detected  by  tests  proved  to  be  adequate,  and  these  only  in  mere  traces, 
between  0.001  and  0.002  per  cent  in  all. 

The  outcome  was  that  100.000  parts  of  pure  silver  produced  157.479 
parts  of  argentic  nitrate.  If,  then,  silver  is  taken  as  107.93,  nitrogen  must 
be  14.037 ;  or  if  silver  is  taken  as  107.880,  nitrogen  must  be  14.008,  oxygen 
being  16.000. 


IV 

The  Molecular  Weight  of  Argentic  Sulphate  and 
THE  Atomic  Weight  of  Sulphur 


By  Theodore  William  Richards  and  Grinnell  Jones 


Contributions  from  the  Chemical  Laboratory  of  Harvard  College 


THE  MOLECULAR  WEIGHT  OF  ARGENTIC  SULPHATE  AND  THE 
ATOMIC  WEIGHT  OF  SULPHUR. 


INTRODUCTION. 

The  atomic  weight  of  sulphur  has  been  investigated  by  many  experi- 
menters; but,  as  will  be  seen,  the  results  are  far  from  concordant.  A 
brief  review  of  these  investigations  forms  the  most  appropriate  introduc- 
tion to  the  present  one.  The  values  in  the  following  list  have  been  recal- 
culated with  modern  figures  for  the  other  atomic  weights  involved:'- 

1814.  Berzelius  (Phil.  Trans.,  104,  20) 32.0 

■*•  1826.  Berzelius   (Pogg.  Ann.,  8,  15)     32.08 

1833.  Turner  (Phil.  Trans.,  123,  539) 32.00 

Do 32.06 

1836.  Thomson  (J.  prak.  Chem.,  8,  370) 32. 

1844.  Erdmann  and  Marchand  (J.  prak.  Chem.  31,  396)    .   .   .  31.99 

1845.  Berzelius  (Lehrbuch,  5th  ed.,  3,  1187) 32.04 

Do 32.16 

1848.    Svanberg  and  Struve  (J.  prak.  Chem.,  44,  320)     ....  32.15 
1851.    Struve   (Liebig.  Ann.,  80,  203) 31.94 

1859.  Dumas  (Ann.  Chim.  et  Phys.  [3],  55,  148) 32.00 

1860.  Stas2  (Bull.  Acad.  Belg.  [2],  10,  153,  322) 32.06 

1878.    Cooke   (Proc.  Amer.  Acad.,   13,  50) 32.137 

Do 31.980 

1891.    Richards  (Proc.  Amer.  Acad.,  26,  268) 32.043 

1898.  Leduc  (Ann.  Chim.  et  Phys.  [7],  15,  94) 32.056 

1899.  D.  Berthelot  (Jour,  de  Phys.  [3],  8,  271) 32.050 

1904.  Jaquerod  and  Pintza  (C.  R.,  139,  129) 32.01 

1905.  Guye    (C.  R.,   140,   1242) 32.065 

1905.    Jaquerod  and  Scheuer  (C.  R.,  140,  1384) 32.036 

The  work  of  the  predecessors  of  Stas  is  now  of  historical  interest  only 
and  does  not  merit  detailed  criticism;  in  recent  times  the  atomic  weight 
of  sulphur  has  rested  chiefly  upon  the  experiments  of  the  famous  Belgian. 

Stas  converted  silver  into  its  sulphide  by  heating  it  in  a  current  of  sul- 
phur vapor  or  hydric  sulphide ;  but  because  he  did  not  know  the  precau- 
tions necessary  for  preparing  perfectly  pure  silver,'  it  is  evident  that  all 
of  his  work  in  which  silver  was  weighed  needs  revision.  Stas  also  reduced 
argentic  sulphate  to  silver  in  a  current  of  hydrogen,  but  in  this  case  he 

iAg=  107.93;  CI  =  35.473;  Pb  =  206.9;  Hg  =  200.0;  Na  =  23.008;  C  =  12.002. 

2Van  der  Plaats,  Ann.  Chim.  et  Phys.  [6],  7,  504  and  526  (1886),  pointed  out  that 
Stas  made  an  error  in  calculating  his  own  data.  Stas  obtained  32.074,  but  32.06  is 
correct. 

8Dumas,  Ann.  Chim.  et  Phys.  [5],  14,  289  (1878);  Richards  and  Wells,  Publica- 
tion Carnegie  Inst.  No.  28,  p.  66  (1905). 

69 


70  MOLECULAR  WEIGHT   OF  ARGENTIC   SULPHATE,   ETC. 

did  not  take  the  very  important  precaution  of  fusing  the  sulphate  before 
weighing  ;^  and  moreover,  the  possibility  that  the  reduced  silver  might  con- 
tain argentic  sulphide  or  sulphate  was  by  no  means  excluded. 

A  brief  review  of  subsequent  work  may  not  be  out  of  place,  although  it 
is  of  a  different  order  of  precision.  Cooke  reduced  argentic  sulphide  in  a 
current  of  hydrogen,  and  concluded  that  the  atomic  weight  of  sulphur 
must  lie  between  the  limits  32.14  and  31.98.  In  summing  up  his  results 
he  stated  that  "this  is  equivalent  to  confirming  the  accepted  value  of 
this  constant,  so  far  as  any  experiments  on  a  scale  less  extensive  than  those 
of  Stas  can  be  of  value  to  this  end."^ 

The  determination  of  the  atomic  weight  of  sulphur  by  Richards  was 
made  incidentally  in  his  work  on  the  atomic  weight  of  copper.  The  ratio 
NajCOg :  NajSOi  gave  as  a  result  S  =  32.075.  If  sodium  is  taken  as 
23,008  instead  of  23.053,  the  result  becomes  S  =  32.043.  No  very  great 
confidence  was  placed  in  these  results  at  the  time,  as  is  shown  by  the  fol- 
lowing sentence :  "The  results  are  hardly  capable  of  deciding  the  present 
uncertainty  in  the  atomic  weight  of  sulphur."^  It  is  probable  that  the 
result  is  too  low,  as  no  proof  could  be  obtained  of  the  thorough  desicca- 
tion of  the  sodic  carbonate. 

Very  recently  numerous  atomic-weight  determinations  have  appeared, 
depending  on  a  purely  physical  method,  based  on  the  assumption  that 
Avogadro's  hypothesis  and  the  simple  gas  law  PV  =  RT  is  strictly  true 
for  gases  when  infinitely  expanded.  Since  it  is  impossible  to  determine  the 
ratios  of  the  densities  of  gases  with  sufficient  accuracy  at  very  low  pres- 
sures, it  is  necessary  to  determine  this  ratio  under  normal  conditions  and 
apply  a  different  correction  in  each  case,  depending  on  the  deviation  of 
the  gas  from  the  simple  gas  law.  It  is  probable  that  the  ratio  of  the  den- 
sities can  be,  and  in  most  cases  has  been,  determined  with  sufficient  per- 
centage accuracy.  On  the  other  hand,  the  correction  is  hypothetical 
and  much  less  certain;  and  accordingly,  the  method  has  but  little  value 
when  the  correction  is  large. 

According  to  Leduc*  the  ratio  between  the  densities  of  sulphur  dioxide 
and  oxygen  is  2.04835.  If  no  correction  is  applied  for  the  imperfection  of 
the  gases,  this  value  leads  to  33.55  for  the  atomic  weight  of  sulphur.  The 
correction  in  this  case  is  unusually  large  and  therefore  must  be  known 
with  a  high  percentage  accuracy  if  the  result  is  to  have  any  value  as 
a  determination  of  atomic  weights.  The  following  are  the  general  meth- 
ods of  calculating  the  correction,  but  none  seems  to  be  of  sufficient  value 
for  the  present  case. 

iRkhards,  Proc.  Amer.  Phil.  Soc,  42,  28  (1903). 
2Cooke.  Proc.  Amer.  Acad.,  13,  52  (1878). 
3Richards,  Proc.  Amer.  Acad.,  26,  269  (1891). 
*Leduc,  C.  R.,  117,  219  (1894). 


INTRODUCTION.  71 

The  first,  called  the  "method  of  corresponding  states"  was  developed 
for  this  purpose  by  Leduc.^  It  depends  on  the  assumption  of  van  der 
Waals  that  two  gases  are  in  corresponding  states  and  deviate  equally  from 
the  hypothetical  perfect  gas  if  their  temperatures  and  pressures  are  equal 
multiples  or  sub-multiples  of  their  critical  temperatures  and  pressures, 
respectively.  In  calculating  the  correcting  factor,  the  compressibility  and 
critical  constants  are  used.  Leduc^  by  this  method  obtained  64.056 
as  the  molecular  weight  of  sulphur  dioxide,  and  34.071  as  the  molecular 
weight  of  hydrogen  sulphide,  and  hence  from  both  S  =■  32.056. 

The  second  method  is  the  "method  of  critical  constants"  as  developed 
by  Guye.^  The  correction  is  calculated  by  the  use  of  van  der  Waals's  equa- 
tion, the  quantities  a  and  b  being  calculated  from  the  critical  constants  of 
the  gases  under  consideration.  In  a  complicated  manner  Guye  applied  a 
correction  to  the  quantities  a  and  b  of  van  der  Waals's  equation,  because 
they  appear  to  vary  slightly  with  the  temperature  and  the  pressure,*  and 
in  this  way  was  obtained  the  value  64.065  as  the  molecular  weight  of  sul- 
phur dioxide;  hence  S  =  32.065. 

The  third  method  is  called  the  "method  of  limiting  density."  It  was 
originated  by  Daniel  Berthelot  in  1898^  and  has  been  used  by  Rayleigh' 
and  Jaquerod.^  It  depends  on  the  experimental  determination  of  the  com- 
pressibilities of  gases  at  pressures  in  the  neighborhood  of  one  atmosphere. 
By  an  extrapolation,  the  "limiting  ratio"  of  the  densities  at  very  low  pres- 
sures can  be  calculated.  This  method  has  reached  its  greatest  perfection 
in  the  hands  of  Lord  Rayleigh,  who  has  shown  that  for  the  permanent 
gases  the  deviation  from  Boyle's  law  only  varies  slightly  with  the  pressure 
and  therefore  the  extrapolation  is  fairly  safe.  This  physical  method  has 
been  applied  with  the  greatest  success  to  the  cases  of  hydrogen,  carbon, 
and,  especially,  nitrogen,  because  in  these  cases  the  correction  is  compar- 
atively small.  Here  also,  however,  the  application  to  sulphur  is  far  less 
satisfactory.  It  is  to  be  regretted  that  Lord  Rayleigh's  work  did  not 
include  compounds  of  this  element. 

Jaquerod  and  Pintza  determined  the  density  of  sulphur  dioxide  at  760 
mm.,  570  mm.  and  380  mm,  pressure,  and  from  these  results  calculated  the 
compressibility.     The  results  were  extrapolated  to  zero  pressure  on  the 

iLeduc,  Ann.  de  Chim.  et  Phys.  [7],  15,  5  (1898). 

2Leduc,  Ann.  Chim.  et  Phys.  [7],  15,  94  (1898). 

sGuye,  C.  R.,  138,  1215  (1904)  ;  Bull.  Soc.  Chim.  [3],  5  Aout  (1905)  ;  Jour.  Chim. 
Phys.,  3,  321  (1905). 

*Guye,  Bull.  Soc.  Chim.  [3],  5  Aout,  p.  xii  (1905). 

sBerthelot,  C.  R.,  126,  954,  1030,  1415,  1501  (1898)  ;  Jour,  de  Phys.  [3],  8,  263 
f  1899) 

«Rayieigh,  Phil.  Trans.  A.,  204,  352  (1905)  ;  A.,  196,  205  (1901),  and  A.,  198,  417 
(1902). 

7jaquerod  and  Pintza,  C.  R.,  139,  129  (1904)  ;  Jaquerod  and  Scheuer,  C.  R., 
140,  1384  (1905). 


72  MOLECULAR  WEIGHT  OF  ARGENTIC  SULPHATE,  ETC. 

assumption  that  the  deviations  from  Boyle^s  law  diminish  as  the  pressure  is 
lowered.  They  obtained  32.01  as  the  atomic  weight  of  sulphur.  After- 
wards Jaquerod  and  Scheuer  determined  the  compressibility  of  sulphur 
dioxide  through  the  ranges  400  to  800  mm.  and  200  to  400  mm.  by  a 
method  similar  to  Lord  Rayleigh's.  They  found,  as  was  to  be  expected, 
that  the  deviation  from  Boyle's  law  was  smaller  at  the  lower  pressure. 
Nevertheless,  in  calculating  the  molecular  weight  of  sulphur  dioxide,  they 
assumed  that  the  deviation  from  Boyle's  law  per  centimeter  of  pressure 
between  0  and  760  mm.  was  the  same  as  between  400  and  800  mm.  They 
obtained  64.036  for  sulphur  dioxide  and  hence  S  =  32.036.  If  their  meas- 
urement of  the  compressibility  through  the  range  200  to  400  mm.  is  used, 
the  result  becomes  32.052,  and  it  seems  probable  that  yet  a  higher  value 
is  the  true  outcome  of  their  experiments.  These  different  conclusions  em- 
phasize the  uncertainty  of  the  method. 

Probably  the  adsorption  of  sulphurous  oxide  and  hydric  sulphide  on  the 
glass  of  the  containing  globes  introduces  an  error  in  both  the  density  and 
compressibility  determinations,  and  hence  in  facts  to  which  the  assumption 
above  mentioned  are  applied.  This  possibility  of  adsorption  seems  to  have 
been  not  sufficiently  heeded  by  any  of  the  experimenters  on  sulphur  com- 
pounds.   It  adds  yet  another  uncertainty  to  the  results. 

The  upshot  of  these  considerations  is  the  conclusion  that  none  of  the 
work  heretofore  done  upon  the  atomic  weight  of  sulphur  decides  its  value. 
Hence  further  investigation  is  imperative.  The  problem,  for  complete 
solution,  must  be  approached  from  many  sides,  and  the  work  must  involve 
many  compounds  of  sulphur.  The  following  contribution  describes  one 
section  of  a  comprehensive  program  by  which  it  is  hoped  the  question  may 
be  answered. 

PRELIMINARY  EXPERIMENTS. 

The  sulphate  of  silver  was  selected  as  the  most  suitable  starting-point 
for  the  present  investigation,  because  it  seemed  probable  that  this  salt 
could  be  prepared  in  a  pure  condition  and  accurately  analyzed.  A  further 
reason  for  studying  argentic  sulphate  lies  in  the  possibility  of  later  com- 
bining the  results  with  some  ratio  involving  the  sulphide  and  thus  fur- 
nishing evidence  on  the  even  more  important  question  as  to  the  atomic 
weight  of  silver.  Balanced  against  the  advantages  is  the  disadvantage 
that  only  about  10  per  cent  of  its  weight  is  sulphur,  causing  the  experi- 
mental errors  to  be  greatly  multiplied  in  the  calculations ;  but  the  advan- 
tages more  than  outweigh  the  disadvantage. 

A  tentative  preliminary  plan  of  operations  was  to  fuse  argentic  sulphate 
in  a  platinum  crucible,  dissolve  it,  precipitate  as  chloride,  and  collect  and 
weigh  the  precipitate.     This  plan  was  thwarted  by  numerous  obstacles. 


PRELIMINARY  EXPERIMENTS.  73 

It  was  found  that  the  sulphate  decomposes  slightly  when  fused,  a  difficulty 
which  was  later  overcome.  Again  the  quantitative  solution  of  the  silver 
sulphate  was  a  very  tedious  process.  Eight  grams  fused  in  a  platinum 
crucible  and  placed  in  1.5  liters  of  water  required  six  weeks  for  its  com- 
plete solution,  even  with  frequent  agitation.  The  action  on  the  glass 
during  this  long  time,  the  danger  of  the  access  of  gaseous  impurity,  and 
the  loss  of  valuable  time  all  made  this  difficulty  a  serious  one. 

Argentic  sulphate  can  be  readily  dissolved  by  placing  twice  its  weight 
of  concentrated  sulphuric  acid  in  the  crucible  and  heating  at  about  300° ; 
on  cooling,  the  acid  sulphate  of  silver  crystallizes  out.^  Upon  adding  water 
to  this  acid  sulphate  the  normal  sulphate  is  formed  as  a  fine  powder,  which 
can  readily  be  washed  into  the  precipitating  flask  and  dissolved.  The 
large  excess  of  sulphuric  acid  thus  introduced  was  far  from  desirable, 
however,  for  it  tends  to  cause  considerable  occlusion  of  argentic  sulphate 
in  the  precipitated  chloride.  In  order  to  test  this  question,  a  solution,  pre- 
pared as  described  above,  and  containing  7.4  grams  of  argentic  sulphate, 
was  precipitated  with  hydrochloric  acid.  The  precipitate  was  washed  by 
decanting  eleven  times  with  very  dilute  hydrochloric  acid  and  then  dis- 
solved in  ammonia  and  diluted.  The  argentic  chloride  was  then  reprecipi- 
tated  with  hydrochloric  acid ;  the  solution  should  contain  all  the  sulphate 
previously  occluded.  After  settling,  this  mixture  was  filtered,  and  the  per- 
fectly clear  solution  was  evaporated  in  a  platinum  dish  over  an  alcohol 
lamp  until  ammonic  chloride  crystallized  out  on  cooling,  the  volume  being 
then  about  0.1  liter.  To  this  were  added  about  10  grams  of  baric  chloride. 
An  undoubted  precipitate  of  baric  sulphate  was  produced,  w^hich  proved 
that  there  is  considerable  occlusion  of  sulphate  by  argentic  chloride  pre- 
cipitated from  solutions  containing  a  large  excess  of  sulphuric  acid. 

For  the  sake  of  comparison,  approximately  the  same  amount  of  argentic 
chloride  prepared  from  nitrate  was  dissolved  in  ammonia,  and  3.5  mg. 
of  sulphate  added.  The  solution  was  treated  exactly  like  the  preceding 
operations,  and  yielded  about  the  same  amount  of  precipitate.  This 
experiment  gives  an  approximate  idea  of  the  extent  of  the  occlusion. 

In  searching  for  a  means  of  wholly  eliminating  the  sulphate  it  was 
found  that  a  small  proportion  of  argentic  sulphate  in  argentic  chloride 
could  be  completely  converted  into  chloride  by  fusion  in  a  current  of  hy- 
drochloric acid  gas.  This  observation  led  to  the  development  of  a  new 
process  in  which  the  entire  reaction  was  carried  out  in  this  manner,  fused 
argentic  sulphate  being  wholly  converted  into  chloride  without  the  need 
of  dissolving  in  water.  Thus  the  operation  was  greatly  simplified,  and  the 
chances  of  error  diminished. 

iSchultz,  Fogg.  Ann.,  133,  143   (1868). 


74  MOLECULAR  WEIGHT  OF  ARGENTIC  SULPHATE^  ETC. 

The  method  of  preparing  the  materials,  the  shape  and  nature  of  the 
apparatus,  and  many  of  the  precautions  and  details  of  manipulation  were 
adopted  only  after  numerous  preliminary  experiments.  A  detailed  account 
of  this  tentative  work  is,  however,  unnecessary,  since  the  important  results 
are  implied  in  the  description  of  the  method  which  was  finally  adopted. 

THE  PREPARATION  OF  PURE  MATERIALS. 

SULPHURIC  ACID. 

The  best  commercial  "chemically  pure"  acid  was  twice  fractionally  dis- 
tilled, using  a  quartz  condenser  and  a  platinum  dish  as  receiver.  Only 
the  middle  fractions  were  used;  17  grams  left  no  visible  or  weighable 
residue  on  evaporation. 

ARGENTIC  SULPHATE. 

Pure  argentic  chloride  residues  from  previous  atomic  weight  investiga- 
tions in  this  laboratory  were  reduced  with  invert  sugar  and  sodic  hydrate. 
The  reduced  silver  was  thoroughly  washed,  and  dissolved  in  nitric  acid; 
and  the  nitrate  was  diluted  and  filtered.  It  was  evaporated  on  the  steam- 
bath  until  saturated,  and  crystallized  by  adding  an  equal  volume  of  con- 
centrated nitric  acid  and  cooling.  The  crystals  were  drained  centrifu- 
gally  as  usual.^  The  crystallization  from  pure  concentrated  nitric  acid 
and  centrifugal  draining  was  repeated  five  times,  using  Jena-glass  vessels. 
It  was  finally  recrystallized  once  more,  using  a  platinum  dish  and  redis- 
tilled nitric  acid.  This  silver  nitrate  was  dissolved  in  a  small  amount  of 
water  in  a  pla'tinum  dish,  and  an  excess  of  the  purest  sulphuric  acid,  diluted 
with  an  equal  volume  of  pure  water,  was  poured  into  it.  The  precipitated 
silver  sulphate  was  drained  in  the  centrifugal  machine.  It  was  then  dis- 
solved in  hot  concentrated  sulphuric  acid  in  a  platinum  dish,  and  the  solu- 
tion was  boiled  for  several  minutes  to  expel  nitric  acid.  On  cooling,  the 
acid  sulphate  crystallized  in  large  crystals.  This  acid  sulphate  was  very 
thoroughly  whirled,  placed  in  a  platinum  dish,  and  treated  with  purest 
water.  Heat  was  evolved  and  the  normal  sulphate  crystallized  out  as  a 
fine  powder.  This  powder  was  washed  by  decantation  with  the  purest 
water  until  the  wash-waters  were  no  longer  acid.^  All  the  water  used  in 
this  work  was  purified  in  the  manner  described  in  previous  communica- 
tions. During  this  washing  the  action  of  light  on  the  wet  silver  sulphate 
produces  a  slight  violet  color.  This,  however,  does  no  harm,  as  any  slight 
decomposition  is  remedied  during  the  subsequent  fusion.     It  was  then 

^Richards,  J.  Amer.  Chem.  Soc,  27,  104  (1905). 

^Unless  this  precaution  is  taken  the  silver  sulphate  can  not  be  dried  thoroughly  in 
the  air-bath ;  in  which  case  it  would  be  impossible  to  clean  properly  the  receiving 
end  of  the  tube  into  which  it  was  afterwards  introduced.     (See  p.  76.) 


THE  PREPARATION  OF  PURE  MATERIALS.  75 

dried  as  much  as  possible  in  the  centrifugal  machine,  and  the  drying  was 
completed  in  an  air-bath  at  110°.  These  operations  were  carried  out  as 
much  as  possible  under  the  protection  of  a  very  large  inverted  funnel,  and 
the  dish  was  kept  covered  with  a  large  watch  glass,  to  avoid  the  introduc- 
tion of  dust.  This  sample  was  called  A  and  was  used  in  the  preliminary 
series  and  in  experiments  4  and  5  of  the  final  series  of  quantitative  experi- 
ments. 

Sample  A  was  dissolved  in  concentrated  sulphuric  acid,  boiled,  crystal- 
lized as  acid  sulphate  by  cooling,  centrifugally  drained,  converted  into 
the  normal  sulphate  by  adding  water,  washed,  and  dried  as  before.  This 
sample  was  called  sample  B.  Since  the  mean  of  the  results  with  this  sam- 
ple is  the  same  as  the  mean  of  experiments  4  and  5,  in  which  sample  A 
was  used,  it  is  evident  that  the  boiling  of  the  sulphuric  acid  solution  fol- 
lowed by  a  crystallization  in  two  different  crystalline  forms  did  not  affect 
the  result ;  therefore  further  purification  seemed  unnecessary. 

HYDROCHLORIC   ACID. 

Hydrochloric  acid,  which  was  only  used  as  a  gas,  was  made  from  two 
sources.  In  experiments  5  and  6  the  hydrochloric  acid  was  made  by  drop- 
ping pure  sulphuric  acid  on  pure  ammonium  chloride.  In  the  other  ex- 
periments the  best  commercial  concentrated  hydrochloric  acid  was  used. 
Richards  and  Wells^  have  shown  that  this  acid  contained  no  other  halogen 
or  arsenic ;  it  was  therefore  suitable  for  our  purpose. 

THE  FUSION  OF  ARGENTIC  SULPHATE. 

Much  thought  was  expended  on  the  devising  of  a  piece  of  apparatus 
which  should  be  suitable  not  only  for  the  fusion  of  argentic  sulphate,  but 
also  for  the  quantitative  conversion  of  the  sulphate  into  the  chloride. 
Finally  the  simple  symmetrical  tube  shown  in  the  diagram  was  found  to  be 

/ ■^^ 


\ / 

Fig.  3.— Quaitz  tube  uied  to  cootain  the  arsentic  salt*.    One-half  actual  aze. 

best.  It  was  the  outcome  of  several  trials  and  suggestions,  and  consisted 
of  a  thin  cylindrical  tube  of  fused  quartz  with  smaller  quartz  tubes  fused 
upon  the  ends.^  The  tube  was  very  light,  weighing  less  than  6  grams.  A 
very  fine  platinum  wire  was  wrapped  many  times  around  the  constricted 

^Richards  and  Wells,  loc.  cit. 

2We  are  greatly  indebted  to  Professor  Baxter  for  his  kindness  in  making  this 
apparatus  from  a  fine  quartz  tube  and  a  quartz  test-tube.  To  Mr.  F.  B.  Coffin  also 
we  are  indebted  for  a  suggestion  which  led  to  a  modification  of  the  original  shape 
of  the  tube. 


^6  MOLECULAR  WEIGHT  OF  ARGENTIC  SULPHATE,  ETC. 

portion  of  the  tube  (^ )  ;  by  pulling  this  the  tube  could  be  readily  rotated. 
It  was  hung  on  the  balance  in  a  horizontal  position  by  means  of  a  platinum 
wire.  The  same  tube  was  used  throughout  the  research,  the  cylindrical 
shape  giving  it  sufficient  strength  to  withstand  all  the  strains  to  which  it 
was  subjected. 

The  tube  remained  remarkably  constant  in  weight  in  spite  of  the  very 
vigorous  treatment  it  received.  During  the  entire  fifteen  experiments  the 
loss  in  weight  was  only  0.41  mg.,  and  during  the  twelve  experiments  of 
the  final  series  the  loss  was  only  0.16  mg.  This  slight  loss  is  probably 
accounted  for  by  a  slight  solubility  of  the  quartz  in  the  concentrated 
ammonia  or  potassic  cyanide  solution  used  to  remove  the  argentic  chloride 
after  each  determination. 

The  sulphate  of  silver  prepared  as  described  above  must  have  contained 
a  little  included  water  and  excess  of  sulphuric  acid,  since  it  had  been  crys- 
tallized from  an  acid  solution,^  The  only  certain  method  of  removing 
mother  liquor  included  within  crystals  is  to  fuse  the  salt.  This  part  of  the 
problem  caused  considerable  difficulty ;  for  it  was  found  that  argentic  sul- 
phate decomposes  slightly  when  fused  in  air,  becoming  yellowish-brown 
in  color.  In  order  to  decrease  this  thermal  dissociation,  the  experiment 
was  made  of  fusing  the  salt  in  a  current  of  dilute  sulphuric  oxide,  with 
complete  success.    The  salt  is  then  pure  white  in  color. 

The  small  crystals  of  argentic  sulphate  were  pushed  into  the  tube  with 
a  platinum  rod,  and  the  narrow  ends  were  freed  from  loosely  adhering 
argentic  sulphate  by  means  of  a  clean  feather  tip,  which  had  been  previously 
washed  with  alcohol  and  ether  to  free  it  from  grease.  The  tube  was  sup- 
ported, on  hooks  of  hard  glass,  in  front  of  the  apparatus  for  delivering 
sulphur  trioxide.  The  very  fine  platinum  wire  needed  for  rotating  it  had 
been  wound  around  the  tube  in  the  first  place. 

The  apparatus  for  delivering  a  current  of  pure  dry  air  or  sulphur  tri- 
oxide or  hydric  chloride  is  shown  diagramatically  in  figure  4.  A  cur- 
rent of  air  was  first  driven  over  red-hot  copper  oxide  (A)  to  remove  or- 
ganic matter,  and  subsequently  through  an  Emmerling  tower  (B)  con- 
taining beads  moistened  with  a  strong  caustic  potash  solution.  This  tower 
was  closed  by  a  rubber  stopper  at  the  top,  which  introduced  no  impurity, 
since  nothing  but  pure  air  passed  over  it.  From  this  point,  however,  the 
apparatus  contained  no  rubber  connections  or  stoppers.  All  stopcocks 
were  lubricated  with  phosphoric  or  sulphuric  acid. 

The  air  was  dried  by  passing  through  two  Emmerling  towers  (C  and  D) 
containing  beads  moistened  with  concentrated  sulphuric  acid.    The  cur- 

iRichards,  Proc.  Amer.  Phil.  Soc,  42,  28  (1903). 


THE  FUSION  OF  ARGENTIC  SULPHATE. 


77 


rent  of  air  could  then  be  either  used  in  a  pure  dry  state  by  passing  through 
L  or  F,  or  else  charged  with  sulphur  trioxide  by  bubbling  twice  through 
fuming  sulphuric  acid  in  the  bottle  E.  This  acid  was  kept  saturated  by 
an  excess  of  solid  sulphur  trioxide.  A  current  of  air  mixed  with  any 
desired  proportion  of  sulphur  trioxide  was  thus  delivered  through  G  into 
the  quartz  tube  H.  The  delivery  tube  (G)  and  the  quartz  tube  {H) 
touched  each  other,  but  no  attempt  was  made  to  secure  a  tight  joint. 


Fig.  4. — ^Apparatus  for  delivering  appropriate  gatet  into  the  quartz  tubes. 


The  argentic  sulphate  contained  in  the  quartz  tube  was  then  fused  with 
a  fish-tail  Bunsen  burner  held  in  the  hand.  A  red  heat  is  required.  When 
all  of  the  sulphate  was  fused,  the  stopcock  (P)  delivering  sulphur  trioxide 
was  closed  and  a  rapid  current  of  air  was  admitted  through  F.  As  soon  as 
the  fumes  of  sulphuric  acid  could  no  longer  be  seen  escaping  from  the  end 
of  the  quartz  tube,  the  flame  was  removed  and  the  tube  was  slowly  rotated 
by  drawing  out  the  platinum  wire  wrapped  around  one  end.  This  causes 
the  argentic  sulphate  to  solidify  in  a  thin  sheet  around  the  tube.  Unless 
care  is  taken  to  keep  the  salt  at  least  1  cm.  distant  from  either  of  the  end 
tubes,  difficulty  is  experienced  during  the  conversion  into  chloride. 

The  final  solidification  in  the  current  of  pure  dry  air  was  carried  out 
very  rapidly,  in  order  to  avoid  decomposition  and  obtain  a  perfectly  white 


78  MOLECULAR  WEIGHT  OF  ARGENTIC  SULPHATE,   ETC. 

sample.  If  the  argentic  sulphate  was  kept  fused  10  or  15  seconds  too  long 
after  the  sulphur  trioxide  current  was  stopped,  a  yellowish-brown  product 
was  obtained.  This  indicates  that  the  salt  contained  either  metallic  silver 
or  argentic  sulphide.  Richards,  Wells,  and  Forbes^  have  shown  that  a 
slight  discoloration  of  a  fused  silver  salt  is  a  very  delicate  test  for  im- 
purity. The  loss  of  weight  due  to  this  discoloration  was  very  slight,  as 
will  be  shown  on  page  79. 

The  entire  tube  was  later  heated  in  a  current  of  pure  air  at  a  tempera- 
ture above  the  boiling-point  of  sulphuric  acid  for  about  5  minutes,  in  order 
to  drive  out  any  possible  accidental  trace  of  acid  in  the  tube.  While  still 
warm  it  was  placed  in  a  desiccator,  and  later  weighed.  The  tube  was  kept 
horizontal  until  the  end  of  the  experiment,  to  avoid  any  chance  of  mechan- 
ical loss. 

The  device  of  spreading  the  salt  in  a  thin  layer  around  the  tube  by  rotat- 
ing the  tube  during  the  cooling  had  four  very  important  advantages: 
First,  the  chance  of  breaking  the  very  fragile  quartz  tube  during  the  solid- 
ification and  cooling  was  greatly  reduced  by  the  more  even  distribution  of 
the  strain;  secondly,  the  salt  was  agitated  and  spread  out  in  a  thin  layer 
while  still  fused  in  a  current  of  pure  air,  thus  facilitating  the  escape  of  any 
possible  trace  of  dissolved  sulphur  trioxide ;  thirdly,  the  cooling  was  made 
more  uniform  and  rapid,  so  that  the  argentic  sulphate  had  no  time  to 
decompose  before  it  had  solidified;  and  fourthly,  a  much  larger  surface 
was  exposed  to  the  later  action  of  the  hydrochloric  acid. 

The  possibility  just  mentioned  that  the  colorless  argentic  sulphate  might 
contain  an  excess  of  sulphur  trioxide  needed  careful  consideration.  Un- 
fortunately a  direct  test  for  acid  seemed  to  be  impracticable,  owing  to  the 
slight  solubility  of  silver  sulphate,  hence  light  upon  the  question  was 
sought  in  several  distinct  ways. 

Weber^  has  found  that  in  order  to  prepare  a  compound  AgjSgOT  argen- 
tic sulphate  must  be  heated  with  sulphur  trioxide  under  pressure  in  a 
sealed  tube.  This,  together  with  the  fact  that  argentic  sulphate  decom- 
poses so  easily  when  fused,  indicates  that  it  does  not  have  a  great  tendency 
to  retain  sulphuric  oxide,  and  that  the  vapor  tension  of  the  trioxide  in  the 
disulphate  is  far  above  that  maintained  in  the  present  experiments.  In 
order  to  obtain  quantitative  evidence,  the  pure  white  sulphate  in  six  of  the 
final  experiments,  after  being  weighed,  was  fused  again  for  about  10 
to  15  seconds  in  order  to  decompose  it  slightly.  This  gave  a  slight  but 
unmistakable  dark  color  to  the  salt.  The  resulting  losses  of  weight  are 
tabulated  on  page  79. 

^Richards  and  Wells,  Carnegie  Inst.  Pub.  28,  31  (1905)  ;  Richards  and  Forbes,  the 
present  publication,  p.  55. 

2 Weber,  Ber.,  17,  2503  (1884). 


THE  FUSION  OF  ARGENTIC  SULPHATE. 
The  Loss  of  Weight  Caused  by  Slight  Decomposition. 


79 


No.  of 

White 

Darkened 

experiment. 

Ab,SO,. 

Ag,SO,. 

Difference. 

9 

5.  27714 

5.27709 

0.00005 

10 

5. 16313 

5. 16302 

0.00011 

11 

5.08383 

5.08377 

0.00006 

12 

5. 13372 

5. 13367 

0.00005 

13 

5. 16148 

5.16138 

0. 00010 

15 
Average... 

5.37436 

5. 37425 

0.00011 

0.00008 

The  loss  in  weight  was  thus  on  the  average  only  0.08  mg.  or  0.0015  per 
cent.  In  seems  certain  that  at  least  part  of  this  loss  in  weight  was  due  to 
a  deficiency  of  either  oxygen  or  sulphur  trioxide  in  the  darkened  sulphate, 
and  not  to  the  loss  of  an  illegitimate  excess  of  the  latter  substance.  There- 
fore, although  it  may  be  a  debatable  question  as  to  whether  it  is  safer  to 
take  the  weight  of  the  white  or  darkened  samples  as  the  true  weight,  it 
seems  probable  that  the  weight  of  the  w:hite  sample  was  more  trustworthy. 
Even  at  the  worst  the  uncertainty  can  not  have  been  greater  than  0.001 
per  cent.  This  result  agrees  with  the  earlier  conclusions  already  cited  con- 
cerning the  obvious  effect  of  very  slight  decomposition  on  the  color  of  sil- 
ver salts.  It  appears  that  in  the  present  case,  as  in  the  others,  the  slight 
stability  of  these  compounds  is  a  real  assistance  in  the  production  of  a 
typical  compound,  instead  of  a  hindrance  to  precise  quantitative  work. 

For  weighing  the  argentic  sulphate,  and  of  course  the  chloride  also, 
the  Troemner  balance  which  had  served  in  many  similar  researches  was 
used.^  The  Sartorius  platinized  brass  weig'hts  were  standardized  by  the 
usual  Harvard  method.^ 

As  usual,  all  weighings  were  made  by  substitution,  a  make-weight  being 
placed  on  the  right-hand  balance  pan.  In  order  to  avoid  as  far  as  possible 
any  error  due  to  changing  meteorological  conditions,  a  substituting  coun- 
terpoise of  the  same  material  and  approximately  the  same  surface  and 
weight  was  used.  Every  weighing  was  repeated,  and  the  successive  values 
seldom  differed  more  than  0.03  mg.  No  difficulty  was  experienced  from 
hygroscopic  adsorption  of  water  by  either  argentic  sulphate  or  argentic 
chloride,  except  on  a  few  days  when  the  air  was  unusually  humid.  In  these 
cases  the  tube  was  heated  to  about  330°  and  allowed  to  remain  in  a  desic- 
cator until  the  conditions  were  more  favorable.  The  tube  was  always 
allowed  to  remain  at  least  three  hours  in  a  desiccator  near  the  balance 


iRichards,  Proc.  Amer.  Acad.,  26,  242  (1891). 
^Richards,  J.,  Amer.  Chem.  Soc,  22,  144  (1900). 


80  MOLECULAR  WEIGHT  OF  ARGENTIC  SULPHATE,   ETC. 

before  making  a  weighing  and  during  this  time  was  covered  by  a  black 
cloth  to  protect  the  sensitive  salts  from  the  light. 

In  order  to  correct  the  weights  to  the  vacuum  standard,  the  specific 
gravity  of  argentic  sulphate  is  needed.  Following  are  the  published  results 
concerning  this  datum: 

Density  of  Argentic  Sulphate. 

5.341.  Karsten,  Schweigger's  J.,  65,  419  (1832), 

5.322.  Playfair  and  Joule,  Mem.  Chem.  Soc,  2,  430  (1845). 

5.410.  Filhol,  Ann.   Chim.   et  Phys.    [3],  21,  417    (1847). 

5.425.  Schroder,  Pogg.  Ann.,  106,  245  (1859). 

554}  Patterson,   Upsala,   Nova   Acta    [3],   9,   35    (1874). 

After  a  study  of  the  original  papers,  the  value  5,45  was  provisionally 
adopted  as  the  most  probable  value;  but,  as  there  was  some  doubt  about 
its  accuracy,  this  was  verified  by  experiment. 

The  density  of  the  toluol  to  be  displaced  by  the  salt  was  determined  by 
means  of  an  Ostwald  pycnometer  at  29,2°  to  be  0.8566  (the  mean  of  3 
concordant  determinations)  ;  6.067  grams  of  previously  fused  silver  sul- 
phate were  found  to  displace  0,9532  gram  of  this  toluol,  and  therefore 
occupied  1.113  ml.  (the  mean  of  two  determinations).  Hence  the  density  of 
argentic  sulphate  is  5.45,  as  supposed.  This  involves  an  additive  correc- 
tion of  0,0000775  gram  to  each  apparent  gram  of  the  salt,  a  value  which 
is  decreased  0,0000011  by  an  increase  of  a  centimeter  of  atmospheric 
pressure,  or  decreased  0,00000026  by  an  increase  of  a  degree  of  tempera- 
ture.   The  correction  to  the  weight  was  applied  accordingly. 

THE  CONVERSION  OF  ARGENTIC  SULPHATE  INTO  CHLORIDE. 

The  next  step  in  the  process  was  the  conversion  of  the  carefully 
weighed  fused  sulphate  into  chloride  by  heating  it  in  a  current  of  dry 
hydrochloric  acid  gas.    This  reaction  has  been  observed  by  Hensgen.^ 

The  hydrochloric  acid  generator  used  for  the  previous  work  was  con- 
structed entirely  out  of  glass.  Pure  ammonic  chloride  or  concentrated 
hydrochloric  acid  was  placed  in  the  flask  (/)  shown  in  figure  4,  and  con- 
centrated sulphuric  acid  was  dropped  upon  it  slowly.  The  gas  was  dried  by 
passing  through  the  tower  (/)  containing  beads  moistened  with  concen- 

^Hensgen,  Recueil  des  Travaux  chimiques  de  Pays-Bas.,  2,  124  (1883),  "Le 
sulfate  d'  argent  absorba  2  mol,  HCl  a  la  temperature  ordinaire,  avec  un  degagement 
de  chaleur  notable,  et  se  changea  completement  en  chlorure.  En  chauffant,  meme 
jusqu  a  300°,  la  reaction  inverse  (observee  par  M.  H.  avec  le  sulfate  de  cuivre)  n'eut 
point  lieu,  mais  I'acide  sulphurique  fut  chasse  completement  par  un  courant  d'air. 
Parmi  les  sels  susdits,  le  sulfate  d'argent  est  celui  qui  d'  apres  les  donnees  thermi- 
ques,  doit  se  changer  en  chlorure  avec  le  plus  grand  degagement  de  chaleur."  (See 
p,  83.) 


THE  CONVERSION  OF  ARGENTIC  SULPHATE  INTO   CHLORIDE.  81 

trated  sulphuric  acid.^  The  gas  then  passed  through  the  stopcock  (K)  to 
the  dehvery  tube  (N).  The  ground-glass  stopper  at  the  top  of  the  tower 
(/)  acted  as  a  safety-valve  when  the  stopcock  (K)  was  closed.  Pure  dry 
air  might  also  be  delivered  at  N  by  means  of  the  stopcock  (L),  in  order  to 
sweep  out  the  excess  of  acid  at  the  conclusion  of  the  reaction. 

The  quartz  tube  containing  the  argentic  sulphate  was  supported  by 
means  of  hooks  of  hard  glass  in  front  of  the  delivery  tube  (N),  as  before, 
without  making  a  tight  joint. 

In  seven  of  the  final  experiments  a  condenser  tube  of  quartz  (O)  was 
placed  over  the  exit  end  of  the  tube  (M)  in  order  to  condense  the  sul- 
phuric acid  and  retain  any  silver  which  might  escape.  As  will  be  shown, 
only  very  small  traces  of  silver  were  found  in  the  distillate.  The  conden- 
sation of  sulphuric  acid  in  the  narrow  tubes  on  the  end  was  prevented  by 
keeping  them  hot  by  means  of  a  small  fixed  fish-tail  burner. 

A  slow  current  of  hydrochloric  acid  was  generated  and  the  tube  warmed 
gently.  The  reaction  took  place  readily  and  quietly,  the  only  difficulty 
being  that  the  argentic  chloride  formed  was  inclined  to  creep  over  the  walls 
of  the  vessel.  This  was  probably  due  to  the  liberated  sulphuric  acid  hav- 
ing dissolved  undecomposed  argentic  sulphate ;  the  acid  sulphate  was  then 
transported  by  surface  tension  and  converted  into  chloride  in  another  place. 
In  two  of  the  experiments  one  of  the  narrow  end  tubes  was  completely 
blocked  up  in  this  manner,  making  a  successful  completion  of  the  experi- 
ment impossible ;  but  in  other  cases  the  difficulty  was  avoided  by  starting 
with  the  argentic  sulphate  in  a  band  in  the  middle  of  the  tube. 

The  sulphuric  acid  must  be  evaporated  at  a  temperature  below  its 
boiling-point  in  order  to  avoid  loss  by  the  projection  of  small  particles. 
The  tube  was  heated  by  a  flame  held  in  the  hand,  the  heating  being  regu- 
lated by  watching  the  escaping  fumes  of  acid  and  also  the  color  of  the 
argentic  chloride  in  the  tube.  It  is  well  known  that  as  the  temperature 
increases,  silver  chloride  acquires  a  deeper  and  deeper  yellow  color;  and 
after  acquiring  the  necessary  experience,  this  change  of  color  proved  very 
helpful  in  regulating  the  temperature. 

After  from  2.5  to  4  hours  no  more  fumes  of  sulphuric  acid  could  be  seen 
issuing  from  the  tube.  The  argentic  chloride  was  then  fused  very  slowly 
and  quietly  and  kept  in  the  fused  state  for  20  minutes  in  a  current  of  hydro- 
chloric acid.     The  tube  was  gently  agitated  in  order  to  expose  a  fresh 

^In  one  of  the  preliminary  experiments  the  hydrochloric  acid  was  not  dried.  The 
sulphuric  acid  first  formed  absorbed  considerable  water,  thus  becoming  diluted  and 
nearly  filling  the  tube  with  liquid  sulphuric  acid  in  which  a  large  part  of  the  silver 
sulphate  dissolved.  After  heating  for  some  time,  the  whole  mass  was  solidified. 
This  gave  a  non-porous  mixture  of  silver  sulphate  and  chloride.  The  hydrochloric 
acid  had  no  appreciable  further  action  until  the  mixture  was  fused,  and  then  the 
action  became  very  vigorous.  The  sulphuric  acid  which  was  formed  boiled,  and 
caused  spattering  and  therefore  danger  of  mechanical  loss. 


83  MOLECULAR  WEIGHT  OF  ARGENTIC  SULPHATE,  ETC. 

surface,  at  intervals  of  about  1  minute,  by  grasping  the  exit  end  with 
platinum  forceps.  The  agitation  can  be  performed  without  danger  of  loss, 
because  of  the  high  surface  tension  of  fused  argentic  chloride.  This  mate- 
rial when  liquefied  does  not  adhere  to  quartz,  and  therefore  is  not  drawn 
into  the  end  tubes  by  capillarity.  At  the  expiration  of  20  minutes  the 
current  of  acid  gas  was  stopped  by  means  of  the  stopcock  (K),  and  a  cur- 
rent of  pure  dry  air  was  passed  through  the  tube  through  L,  the  argentic 
chloride  being  still  maintained  in  the  liquid  state  for  at  least  5  minutes 
with  occasional  agitation.  The  tube  was  rotated  while  subsequently  cool- 
ing in  a  similar  manner  to  that  employed  with  the  sulphate. 

The  condensed  sulphuric  acid  evolved  by  the  reaction  and  the  condenser 
tube  were  tested  for  silver  by  means  of  the  nephelometer.^  The  acid  was 
transferred  to  a  small  flask  and  the  tube  was  rinsed  with  ammonia,  which 
was  added  to  the  acid.  The  excess  of  ammonia  was  then  neutralized  with 
nitric  acid,  and  hydrochloric  acid  was  added  to  precipitate  any  trace  of 
silver  present  as  an  opalescent  cloud  of  argentic  chloride.  This  opalescence 
was  compared  in  the  nephelometer  with  a  standard  which  had  been  made 
in  a  manner  very  similar  to  the  unknown  solution.^  A  measured  volume  of 
a  standard  silver  solution  was  taken ;  to  it  were  added  approximately  the 
same  amounts  of  sulphuric  acid,  ammonia,  and  nitric  and  hydrochloric 
acids  as  were  present  in  the  unknown  solution ;  and  it  was  made  up  to  the 
same  volume  (about  30  ml.). 

The  greatest  amount  of  argentic  chloride  thus  found  in  any  experiment 
was  0.00009  gram  and  the  average  amount  0.00004  gram.  The  trace 
found  in  this  way  was  of  course  added  to  the  weight  of  the  silver  chloride. 
In  three  of  the  experiments  (Nos.  4,  5,  and  7)  this  determination  was  not 
made,  but  the  average  amount  is  added  in  these  cases.  The  probable  rea- 
son for  the  escape  of  this  trace  of  silver  will  be  discussed  later. 

The  question  as  to  whether  or  not  this  reaction  is  complete  is,  of  course, 
of  fundamental  importance.  There  are  four  pieces  of  evidence  bearing 
on  this  point. 

In  the  first  place,  the  argentic  chloride  was  fused  for  twenty  minutes  in 
a  current  of  almost  pure  hydrochloric  acid.  Since  the  temperature  was 
far  above  the  boiling-point  of  sulphuric  acid,  this  product  of  the  reaction 
was  driven  off  as  fast  as  formed.  Therefore  according  to  the  law  of 
concentration-effect  it  is  to  be  expected  that  through  the  agency  of  the 
continually  renewed  hydrochloric  acid  all  the  argentic  sulphate  would  be 
decomposed  and  all  the  sulphuric  acid  would  be  driven  oflf.  This  would 
be  hastened  by  the  fact  that  fresh  surfaces  were  continually  exposed 
through  agitation. 

iRichards  and  Wells,  Amer.  Chem.  Jour.,  31,  235  (1904). 
2Jlichards,  Amer.  Chem.  Jour.,  35,  510  (1906), 


THE  CONVERSION  OF  ARGENTIC  SULPHATE  INTO  CHLORIDE. 


83 


Again,  in  this  case  the  law  of  concentration-effect  is  assisted  by  the  rela- 
tive affinity,  indicated  approximately  by  the  large  amount  of  heat  evolved 
by  the  reaction.  It  can  be  calculated  from  Thomsen's  data  that  the  reac- 
tion gives  out  40,400  grams  calories,^  or  170  kilojoules.  Rarely,  if  ever,  is 
the  difference  between  total-energy  and  free-energy  changes  in  a  reaction 
of  this  kind  as  great  as  this,  hence  it  is  safe  to  infer  that  there  is  a  con- 
siderable preponderance  of  driving  tendency  in  the  desired  direction,  aris- 
ing from  the  mutual  affinities  concerned. 

Thirdly,  constancy  of  the  weight  on  continued  treatment  indicated  the 
completion  of  the  reaction.  In  seven  of  the  experiments  the  argentic 
chloride  was  fused  a  second  time  in  hydrochloric  acid  for  15  minutes  with 
occasional  agitation,  followed  by  5  minutes  in  air.  The  following  table 
g^ves  the  changes  in  weight  found  in  this  way : 

The  Effect  of  Continued  Treatment  with  Hydrochloric  Acid. 


No.  of 

AgCl,  first 

AgCl,  second 

experiment. 

weighing. 

weighing. 

Difference. 

6 

4. 67812 

4. 67809 

—  0. 00003 

7 

4. 93119 

4. 93118 

—  0. 00001 

11 

4. 67373 

4. 67375 

+  0.00002 

13 

4.  74491 

4. 74489 

-0.00002 

14 

4.77995 

4. 77990 

—  0.00005 

15 
Average... 

4. 94088 

4.94088 

+  0. 00000 

—  0.000015 

The  constancy  in  weight  was  thus  entirely  satisfactory,  the  average  loss 
being  only  0.0003  per  cent  of  the  weight  of  the  chloride. 

This  experiment,  however,  does  not  absolutely  preclude  the  possibility 
that  a  small  but  constant  amount  of  sulphate  may  remain.  In  order  to  test 
this  question  4.90  grams  of  argentic  chloride  which  had  never  been  con- 
taminated with  sulphate  was  fused  in  the  tube,  and  then  0.00479  gram  of 
pure  argentic  sulphate  was  added  and  thoroughly  mixed  with  the  chloride 
by  fusion.    On  cooling  the  appearance  was  very  different  from  the  pure 


382.) 
255.) 


^Thomsen,  Thermochemische  Untersuchungen : 
2Ag  -f  CI2  =  2AgCl  +  58760.     (Vol.  3,  p.  381.) 
Ag,S04  =  2Ag+02  +  S02  — 96200.  (Vol.  3,  p. 
S0»  +  O2  +  Ha  =  HjSO*  -f- 121840.     (Vol.  2,  p. 
2HC1=:H2  +  Cl2  — 44000.     (Vol.  2,  p.  114.) 
Therefore,   AgjSO*  +  2HC1    (gaseous)  =  2AgCl  +  H2SO*    (Liquid)  +  40,400. 
A  different  set  of  equations  gave  40,200.     The  result,  of  course,  only  applies  to  18°. 
In  this  connection  it  is  worth  while  to  note  that  AgaSO*  +  CI2  =  2AgCl -f- SO2  + 
Oi  —  37440  cal. ;   which  indicates  that  chlorine  would  not  be  as  suitable  for  our 
purpose  as  hydrochloric  acid.     This  expectation  is  confirmed  by  the  experience  of 
Krutwig,  Ber.,  14,  306  (1881):  "Die  Einwirkung  ist  hier  (chlorine  on  silver  sul- 
phate) keine  direkte;  nur  bei  sehr  hoher  Temperatur,  nachdem  das  Salz  geschmol- 
zen  ist  und  sich  zersetz,  giebt  es  schweflige  Saiire,  Chlorsilber  und  Sauerstoflf  ab." 


84  MOLECULAR  WEIGHT  OF  ARGENTIC  SULPHATE,  ETC. 

chloride,  the  mass  being  translucent  or  almost  opaque,  instead  of  transpar- 
ent. It  was  then  fused  in  a  current  of  hydrochloric  acid  for  twenty  minutes 
after  the  sulphuric  acid  could  no  longer  be  seen  escaping  from  the  tube,  as 
before.  On  the  assumption  that  the  sulphate  was  converted  completely 
into  chloride  the  loss  in  weight  would  have  been  0.00039  gram,  while  the 
actual  loss  in  weight  was  0.00044  gram.  The  reaction  was  evidently  com- 
plete, and  a  fourth  argument  was  added  to  the  other  reasons  for  believ- 
ing that  the  process  should  yield  satisfactory  results. 

It  will  be  recalled  that  the  narrow  end  tubes  were  kept  very  hot  by  small 
stationary  burners  in  order  to  prevent  the  condensation  of  sulphuric  acid. 
At  the  close  of  several  of  the  experiments  a  very  slight  sublimate  was 
observed  on  the  portion  of  the  narrow  tubes  which  was  kept  somewhat 
cooler  by  the  protection  of  the  supporting  hooks.  This  sublimate,  although 
never  large  in  amount,  appeared  largest  in  experiments  6  and  10.  There 
was  a  smaller  amount  in  experiments  7,  11,  and  14,  but  none  worthy  of 
consideration  in  experiments  4,  5,  12,  13,  and  15.  In  experiments  10  and 
14  a  slight  amount  of  the  sublimate  was  visible  in  the  condenser  also,  near 
the  end  of  the  tube. 

Although  the  most  rational  explanation  of  this  trace  of  sublimate,  which 
was  usually  too  slight  to  be  weighable,  was  probably  to  be  found  in  the 
assumption  that  it  was  argentic  chloride  and  therefore  entirely  without  sin- 
ister meaning  except  as  suggesting  the  risk  of  the  loss  of  other  traces, 
the  matter  was  studied  further.  Careful  tests  for  arsenic^  and  copper  were 
made  with  negative  results,  and  the  hydrochloric  acid  was  demonstrated 
to  contain  no  trace  of  anything  which  could  be  deposited  in  a  red-hot 
quartz  tube.  On  the  other  hand,  the  sublimate  was  soluble  in  ammonia 
and  behaved  in  every  way  like  argentic  chloride,  so  that  its  nature  was 
considered  as  nearly  proved  as  was  possible  with  such  a  minute  amount  of 
material.  Having  settled  the  nature  of  the  sublimate,  its  source  and  signi- 
ficance must  be  traced.  Evidently  it  could  not  have  come  from  the  main 
body  of  the  silver  sulphate,  because  it  appeared  at  the  very  beginning  of  the 
experiment,  as  soon  as  the  current  of  hydrochloric  acid  was  started  and 
the  end  tubes  heated,  before  the  heat  was  applied  to  the  main  body  of  the 
argentic  sulphate.  Moreover,  the  mass  of  the  argentic  chloride  was  never 
heated  to  a  temperature  high  enough  to  volatilize  weighable  amounts  of 
this  substance,  as  was  shown  by  the  constancy  of  weight  on  continued 
heating  in  a  current  of  gas.^ 

In  view  of  these  considerations,  it  seemed  probable  that  a  few  invisible 
crystals  of  argentic  sulphate  had  been  left  in  the  end  tubes  by  the  feather 

^This  test  was  kindly  made  by  Mr.  O.  F.  Black. 

2See  the  preceding  description;  also  Richards  and  Wells,  loc.  cit.,  p.  60;  Baxter, 
Proc.  Amer.  Acad.,  41,  83  (1905). 


THE  CONVERSION  OF  ARGENTIC  SULPHATE  INTO  CHLORIDE.  85 

used  in  cleaning  it,  or  carried  into  the  end  tube  by  the  current  of  sulphur 
trioxide  before  fusion.  As  soon  as  the  hydrochloric  acid  was  admitted  and 
the  end  tubes  heated,  these  small  invisible  crystals  of  silver  sulphate  must 
have  been  at  once  converted  into  chloride  and  sublimed  to  the  cooler  por- 
tion of  the  tube  —  for  the  end  tubes  were  usually  raised  to  a  red  heat.  This 
explanation  is  consistent  with  the  frequent  absence  of  any  significant  sub- 
limate, especially  in  the  case  of  experiment  15.  In  this  experiment  the 
tube  had  been  dusted  and  treated  with  particular  care,  in  order  to  test  the 
point. 

Because  the  sublimate  was  weighed  in  the  tube  which  had  previously 
contained  the  sulphate,  its  presence  could  not  affect  the  accuracy  of  the 
results  even  if  it  had  been  weighable.  Nevertheless  the  suggestion  that 
another  portion  might  have  been  carried  out  of  the  tube  was  worth  con- 
sidering. Doubts  on  this  point  were  set  at  rest  by  the  analysis  of  the  con- 
tents of  the  condenser  tube  which  received  the  volatile  products  of  the 
reaction.  The  average  amount  of  silver  found  in  this  tube  was  less  than 
0.001  per  cent  of  that  taken  in  each  experiment,  and  the  small  appropriate 
correction  was  easily  applied. 

Although  it  was  not  probable  that  argentic  chloride  which  had  been 
fused  in  air  for  five  minutes  still  retained  any  dissolved  hydrochloric  acid, 
this  point  also  was  tested.  In  experiment  13,  the  silver  chloride  after  the 
first  heating  in  hydrochloric  acid  and  fusion  in  air  for  five  minutes  as 
usual,  weighed  4.74491  grams.  After  the  second  fusion  in  hydrochloric 
acid,  and  finally  in  air,  the  weight  was  4.74489  grams.  After  another 
fusion  for  twenty  minutes  in  a  current  of  pure  air,  the  weight  was  4.74493 
grams.  These  slight  changes  in  weight  can  only  be  ascribed  to  errors  in 
the  weighing ;  the  outcome  shows  that  the  argentic  chloride  after  the  usual 
treatment  did  not  retain  any  dissolved  hydrochloric  acid.  Richard  and 
Wells  have  already  shown  that  it  does  not  dissolve  weighable  amounts 
of  air.^ 

In  still  another  case  the  outside  of  the  tube  was  washed  with  water  to 
make  sure  that  nothing  had  deposited  on  it  during  the  long  exposure  to 
the  flame  and  acid.  The  loss  in  weight  was  only  0.02  mg.,  which  again  is 
not  greater  than  the  possible  error  in  weighing. 

^Richards  and  Wells,  loc.  cit.,  p.  60. 


86 


MOLECULAR  WEIGHT  OF  ARGENTIC  SULPHATE.  ETC. 


THE  FINAL  RESULTS. 

Having  shown  the  feasibility  of  the  plan  of  operations  and  gained  prac- 
tice by  three  preliminary  experiments,  a  final  series  was  undertaken,  whose 
outcome  is  detailed  below. 

Sample  A  of  argentic  sulphate  was  used  in  experiments  4  and  5  and 
sample  B  in  the  remainder.  In  experiments  5  and  6  the  hydrochloric  acid 
was  generated  from  ammonic  chloride,  and  in  the  other  experiments  from 
concentrated  hydrochloric  acid.  The  corrected  weight  of  argentic  chloride 
was  obtained  by  adding  the  trace  found  in  the  condenser  to  the  average 
weight  after  fusion  in  hydrochloric  acid.  In  experiments  4,  5,  and  7  the 
correction  for  the  argentic  chloride  in  the  condenser  was  not  determined 
directly,  but  the  mean  of  the  other  determinations  (0.00004  gram)  was 
added. 

In  experiments  8  and  9  the  narrow  exit  tube  became  completely  blocked 
by  the  creeping  of  the  argentic  chloride.  This  made  it  necessary  to  fuse 
the  solid  while  there  was  still  present  considerable  argentic  sulphate ;  and 
therefore  the  sulphuric  acid  boiled  and  material  was  lost  by  being  pro- 
jected out  of  the  tube.  The  first  of  these  was  rejected,  and  the  second  not 
finished.  All  the  other  determinations  are  recorded  in  full  in  the  table. 
The  weighings  have  of  course  been  corrected  to  the  vacuum  standard. 

The  Quantitative  Conversion  of  Argentic  Sulphate  to  Chloride. 


No.  of 
experiment. 

Weight  of  color- 
less   AB2S04 
in  vacuum. 

Weight  of 

total  AgCl  in 

vacuum. 

Parts  of  AgCl 
obtained  from 
100.000  parts  of 

Ae^so^. 

4 

5. 21962 

4. 79859 

91. 934 

5 

5. 27924 

4. 86330 

91.932 

6 

5. 08853 

4.67810 

91.934 

7 

5. 36381 

4.93118 

91.934 

10 

5. 16313 

4. 74668 

91.934 

11 

5. 08383 

4. 67374 

91. 933 

12 

5. 13372 

4. 71946 

91.931 

13 

5. 16148 

4. 74490 

91.929 

14 

5. 19919 

4. 77992 

91.936 

15 
Average... 

5. 37436 

4. 94088 

91.934 

91.933 

Thus  100.000  parts  of  colorless  argentic  sulphate  were  found  to  yield 
91.933  parts  of  argentic  chloride,  with  a  vanishingly  small  "probable 
error."  If  the  weight  of  the  darkened  samples  are  used  in  the  calculation 
the  result  becomes  91.934,  a  value  which  certainly  represents  the  maxi- 
mum. To  have  reduced  the  chief  uncertainty  to  within  such  narrow  limits 
was  to  have  solved  the  problem  as  far  as  it  need  be  solved  at  present. 
The  interpretation  of  the  result  alone  remains. 


THE  FINAL  RESULTS.  87 

111  comparing  this  result  with  that  of  Stas  on  argentic  sulphate,  it  may 
be  noted  that  Stas  found  in  silver  sulphate  69.203  per  cent  of  silver.  Be- 
cause Richards  and  Wells  found  in  argentic  chloride       '   — 100  =  75.2633 

per  cent,  it  is  clear  that  our  results  indicate  0.91933  X  75.2632  =  69.192 
per  cent  of  silver  in  silver  sulphate,  or  0.011  less  than  Stas's,  one  is  forced 
to  the  conclusion  that  Stas's  argentic  sulphate  was  not  completely  reduced 
by  hydrogen,  and  that  his  silver  obtained  in  this  way  was  no  purer  than 
the  silver  used  in  his  other  work.  The  only  test  of  complete  reduction 
used  at  the  time  was  the  solution  of  the  residual  metal  in  nitric  acid ;  but 
this  test  could  not  reveal  undecomposed  sulphate  and  might  not  reveal 
traces  of  sulphide. 

THE  ATOMIC  WEIGHT  OF  SULPHUR. 

The  interpretation  of  the  new  results  is  very  similar  to  that  already  dis- 
cussed in  the  preceding  paper  on  nitrogen  and  silver.  In  this  case,  as  in 
that,  there  are  two  uncertain  ratios ;  and  one  of  these  ratios,  namely,  that 
of  silver  to  oxygen,  occurs  in  each.  In  the  present  case,  the  other  uncer- 
tain ratio  is  that  of  sulphur  to  oxygen,  while  in  the  former  case  the  other 
uncertain  ratio  was  that  of  nitrogen  to  oxygen. 

In  order  to  obtain  a  complete  solution  of  the  numerical  relations  in  either 
of  these  cases,  it  is  therefore  obviously  necessary  to  obtain  another  series 
of  results,  bringing  in  such  a  ratio  as  that  of  silver  to  sulphur,  or  oxygen 
to  sulphur,  or  chlorine  to  oxygen.  Such  an  additional  ratio  is  not  known 
at  present  with  modem  accuracy.  Because  of  the  fact  that  silver  and 
oxygen  are  concerned  in  each  of  the  cases,  a  single  new  result,  properly 
chosen,  will  solve  both  problems  at  once ;  but  of  course  many  new  results 
with  different  compounds,  confirming  one  another,  are  greatly  to  be 
desired.  As  has  been  said  in  the  foregoing  paper,  some  of  these  are 
already  in  the  process  of  determination  at  Harvard  College,  and  it  is 
intended  to  pursue  the  matter  further  at  the  University  of  Berlin  as  well. 

For  the  present  it  is  necessary  to  state  the  interpretation  of  the  results 
in  a  conditional  manner,  assuming  various  possible  ratios  between  oxygen 
and  silver,  and  stating  the  corresponding  values  for  sulphur.  In  the  fu- 
ture, when  the  assumed  relationship  is  replaced  by  knowledge  of  the  facts, 
intelligent  choice  can  be  made  between  the  alternatives. 

If  oxygen  is  taken  as  16.000,  the  following  table  gives  the  atomic 
weights  of  sulphur  corresponding  to  the  several  atomic  weights  of  silver. 

Atomic  Weight  of  Sulphur, 

If  Ag=:  107.930  and  CI  =  35.473,  S  =  32.1 13 
If  Ag=  107.890  and  CI  =  35.460,  S  =  32.078 
If  Ag  =  107.880  and  CI  =  35.457,   S  =  32.069 


88  MOLECULAR   WEIGHT   OF   ARGENTIC   SULPHATE,    ETC. 

The  lowest  value  in  this  case,  as  well  as  in  the  case  of  nitrogen,  is  the 
one  supported  by  the  recent  work  on  the  densities  of  gases. 

In  conclusion,  it  is  a  pleasure  to  acknowledge  the  generous  assistance 
of  the  Carnegie  Institution  of  Washington,  without  which  the  present 
work  could  not  have  been  performed. 

SUMMARY. 

The  most  important  results  of  the  research  may  be  briefly  summed  up  as 
follows : 

(1)  A  method  for  the  preparation  of  pure  argentic  sulphate  was  devised. 

(2)  The  specific  gravity  of  argentic  sulphate  (previously  fused)  was 
found  to  be  5.45. 

(3)  Indication  was  obtained  that  Stas  was  unable  wholly  to  reduce  sil- 
ver sulphate  in  hydrogen. 

(4)  Argentic  sulphate  was  found  to  be  occluded  by  silver  chloride  from 
solutions  containing  an  excess  of  sulphuric  acid. 

(5)  It  was  proved  that  argentic  sulphate  can  be  completely  converted 
into  silver  chloride  by  heating  in  a  current  of  hydrochloric  acid  gas. 

(6)  100.000  parts  of  argentic  sulphate  were  thus  found  to  yield  91.933 
parts  of  argentic  chloride. 

(7)  The  atomic  weight  of  sulphur  as  calculated  from  this  ratio,  if 
oxygen  is  taken  as  16.000,  with  several  assumed  values  for  silver  is: 

Ag=  107.93  S  =  32.113 

Ag  =  107.89  S  =  32.078 

Ag=  107.88  S  =  32.069 

Attention  is  called  also  to  the  summaries  of  the  three  previous  papers 
on  pages  24,  44,  and  65. 


^/^■^ryAmMf^yimiff/M:. 


